For starters I probably wouldn't notice it at all (I once managed to withdraw 80 euros from the cash distributor and leave without taking the money, this is how abysmal my Se function is), but supposing I did:
"Huh? 2 bucks? Whatever, I'm not gonna risk any trouble with its possible owner for just 2 bucks."
If it was more (like, if I was the guy who found those 80 euros at the cash distributor) it might be different. If I could be sure I wouldn't get into trouble I'd probably take and keep it, no guilt involved. "Someone's bound to take it; why not me?"
However if the owner then came and asked what happened to their money then I'd give it back, even if he /she didn't know I was the one who took it.