If there is no data on the secret identity say Peter Parker on file then all the evidence you collect on Spiderman is useless as there is nothing to compare it to. I am sure there are terabytes of info on all superheroes but without the ability to compare it to something its all pretty useless. 
 
Now in some instances someone like Oracle may have a worm program running that deletes all computer references to certain superheros as soon as it is entered. Deleteing the actual physical evidence would be harder but how many people today look beyond the computer to see if somethng is there.

I'm sorry, I was thrown off the Black Cat's undies, but yeah this always kinda bothered me too.

Ugh.  You mentioned possibly the worst Superman movie ever.  The hair was cut by scissors, for crying out loud LOL.  I do find this an interesting idea though.  I think that the investigators have a good understanding of the benefits of having superheroes and may tend to look the other way.  Besides, though, even with all that evidence, would there necessarily be a database to compare against if the heroes had no criminal records ?  I can feel the paranoia rising up here

I've always wondered about this myself. I mean in most superhero comics after a particularly violent battle (spider-man for example) the heroes usually left bloody with their costume in shreds. Surely a half CSI would have no problem finding blood or whatever at the scene and figuring it out.
I know Icemizer mentioned they may not have data on the hero's secret identity but that would just mean one small slip up with the law and their whole identity is compromised, and lets face it Marvel heroes secret identities have a knack of getting framed for things...

 @G-Man: As a forensic scientist i can tell you that i agree they would have  millions of potentially tell tale evidence laying around from all there heroic exploits, but much of it would be made useless by the fact they cant tie the evidence to the person since a very high percentage of superheroes dont have criminal records. For the people who have ex military history and criminal records they could be indetified easy if evidence is weatheed or contaminated before testing thou.
 
Always suprises me that batman hasnt been id'd since he often leaves blood samples from various cuts and wounds he has.  I'am sure Bruce would have had medical files from when he was injured as kid ,not to even mention if blood was  tested would show simularties to the his parents blood which is probably saved on file as evidence somewhere from their shooting.
 
Also in terms of  removing potential evidence by altering costume you would need to consider so many aspects since can even gather some pyschical aspects of a suspect  from a simple shoe print , hair  uniform fibres skin flakes etc are all again very hard to 100% nulify thats why  people say there is no perfect crime its near impossible to remove 100% of trace evidence.
 
Finally CSI is so floored in term os actual practise its brilliant , my first year of uni we had a lecture titled (why csi is wrong about nearly everything)  

Mr. Kent's DNA should raise a few eyebrows if it's ever tested. Mr. Parker's wouldn't exactly be text book either with all the arachnid in there. 

They're just lucky they don't live around here... Imagine trying to keep your secret identity a secret, when there's a CCTV camera on every street corner, following you around, watching you change costumes & then trailing you back home...    

What would be the point in investigating superheroes in the first place? They haven't done anything wrong.

I would expect the heroes to be careful for that but you're right they're not exactly being careful if you are leaving fingerprints on rings...

I've noticed this as well, but in case you haven't noticed, cops in the "comic" world aren't always depicted as superior in intelligence. Which is why they need heroes in the first place. 

"They're just lucky they don't live around here... Imagine trying to keep your secret identity a secret, when there's a CCTV camera on every street corner, following you around, watching you change costumes & then trailing you back home...     "

" They're just lucky they don't live around here... Imagine trying to keep your secret identity a secret, when there's a CCTV camera on every street corner, following you around, watching you change costumes & then trailing you back home...     "

You have to be in the system for prints or dna to mean anything. What are they going to match it up with?  All of these forensics shows are pure bullshit and nobody seems to realize that.

I know Bruce has set up tampered files for the Bat Family and most of the JLA to protect them 

speaking of hair.... how does Superman get his hair cut if he is invulnerable? And how does Wolverine too? does his hair have a healing factor?

The reason CSI has never found any evidence?
 
Comic book logic.

Maybe its like Gaticca (spelling?) we just don't see the hours that the heroes spend cleaning up any traces of their hair, skin cells, and fingerprints.  We also don't see them use the restroom either.
I think that to connect someone with the hero identity you need to have both DNA samples.  If the character has a record I think they already have the sample.  But then I think you have to think of that person too.  (I am not a CSI or a scientist of any sort so I could be completely wrong).
But I think if no one thinks of the other person, they wont get the DNA to match the hero's DNA to.  They can't run it through a computer and have it pop up, "This is Peter Parker's DNA."  (I don't think).  They have to compare it to another DNA sample to see if its a match, and maybe there isn't enough of a match to make a case or something.  (again, still not a CSI person).
Also some comics have brought up the fact that the hero has sometimes gotten in the way of the investigation.
Some criminals get out on technicalities because the hero beat them up, and I am sure because other evidence left by the hero tampered with the evidence that would have lead to the bad guy being convicted.
 Also, maybe sometimes the cops look the other way or throw out the evidence the hero has left or something.

Actually on the converse of this, it's always amusing that there's a multitude of clues villains leave behind that police investigators miss, but even after massive brawls there's rarely anything left behind that police or villains find of the heroes.

Well, Spider-Man has a physic blindspot.  But I can't say for the others.

yea not only that....But even if they did have Info on the people to expose them they would probably just become criminals for technically being Vigilante's b/c they are not Certified Police Officers and I don't think they would arrest a the only group of People able to stop the overwhelming Group of Supervillans that Come out of nowhere  for ex. They see proffesor X  take down a Bad Guy....But they aren't gonna Arrest him and Take hold of his property B/c he is the only guy who can handle the evil mutants and not too mention then they would have to find some where to place the School full of mutants 

A lot of what is on CSI is bull sugar  Finger prints from people not in the system would be useless and finger prints are very much exaggerated in Hollywood they rarely get a good print unless it's on glass or something and that's if they even bother. DNA from a super hero would just mean that a super hero had been there. They'd have no idea which one or where they were. 
 
Superman's hair is too strong to just fall off. That's not ad hoc, I am basing that on his wind speeds and how dense the rest of his body is. It won't even burn from fire. It would also be hard to tell it was superman's hair unless you pulled on ever hair you happened to find. Superman 4 had a strand of his hair (freely given by superman) in a museum holding up a 1,000 lbs weight or something. Luther went there and stole it. It never made sense to me how he cut it with scissors I guess they had kryptonite on them. 
 
I wonder why more bad guys don't just follow them home. That's how the Green Goblin figured out Spider-man's identity and how Ana K. deduced it to either Parker or his roommate. 
 
You'd figure a guy like Bataman who is 6'4 and must be or be connected to a billionaire who lives in the Gotham area couldn't be that hard to deduce but they never get him. 
 
Superman, good grief is Clark even much of a disguise anyway? A pair of glasses. And he works with investigative journalist at least three of whom talk to both he and the Clark version up close pretty regularly enough to know what they look like.  Same height same face, both muscular as hell. How hard can that be to deduce? 
 
Understandably someone like Thor or Captain Marvel who completely change would be harder to deduce  But there are tons that would be easy to just follow home. Or figure out because they work with them.

Ok in situations like this I honestly think places like SHIELD come into play, that and the fact that I believe that some law enforcements places look after the hero community in this aspect. I mean honestly when Galactus comes down to earth do you think the cops want to rush out to fight him, nope they call out the heroes to do it, so why not help them out and lose some paperwork or drop  some evidence here and there.

"Mr. Kent's DNA should raise a few eyebrows if it's ever tested. Mr. Parker's wouldn't exactly be text book either with all the arachnid in there.  "

maybe cuz they're lazy

I actually think in Superman 4 the hair was connected to wires itself and that's what Lex cut. To be honest its been so long since I've seen it, but for some reason when I think about it I see two thicker pieces at the ends of the strand of hair. I could be wrong though and he just cut the hair.

I'm sure the government knows who all the superheroes are. The cops obviously aren't up to snuff or just plain don't wanna know in most comic books. 

i think that the radioactivity in spiderman's blood kills all dna traces. 

but it doesn't make any sense at all.  they would all be cought really fast if there dna ever got in the system--which i guess it isn't 

for dc im not too sure but for marveli always thought sheild had a clean up crew 

" @Chaos Burn:  I remember once Superman using his heat vision bounced off a piece of Kryptonian metal to shave. Might've been in an 80s comic. "