While playing FO3, every time I come across a Raider lair, there is a neon-greenish spray-painted symbol on doors, walls, etc. There are other colors as well, but I don't remember them at the moment. My question is, is there a specific symbol that represents a gang of Raiders? Or is it just random graffiti?

I think its just random grafitti. I've noticed that every baddy in fallout 3 has some enviormental decorations associated with them.

Hmm.....are you sure about that? I figured they were random as well, but it would be neat if there was some backstory behind it.

nah, all the raider holes have those signs, just circles and other....stuff.

probably some drunk or jet high raider gor hold of a spraycan with neon paint.

Yes, you're probably right. Unfortunately enough.

yeah, it wouldve been cool if you could recognise different raider teams by their symbols on the walls, like "oh sh*t! this is Khans/East Side Slingers/Warriors/Surf Nazis/Cobra Kai territory! better scram!".

^ I agree with that, but in the end it does'nt really matter if its a specific gang of raiders, because the only good raider, is a dead raider.

Sounds like it would be a good candidate for a mod whenever they release the construction set.

Hmm, In Fallout 2 there were the Khans... their symbols was a Skull with a Ghengis Khan Beard.... maybe the ones in fallout 3 have backstory too, just have to look... hard

Well, there's the circular...thing (whatever the hell that is) and something else that is unmistakably a mushroom cloud. Too bad there's no actual Raider gang/tribal names - I could imagine the baddest bunch calling themselves Deathclaws (having a totemic skull in their HQ). My screenname would be a good Raider gang name, for that matter - Yao Guai Slayers, rulers of the south side!
I still wonder how they can stand to live with the stench of rotting dismembered corpses in and around their lairs, but that's another issue. (I imagine heavy use of Psycho/Jet cocktails washed down with stupid amounts of whiskey and beer might have something to do with it.)