It was all in one clump of bills. Someone had clearly dropped all their money.

One time, we found $60, and we approached the only people on the same sidewalk and asked them if they'd dropped any money. They said they had, and we gave them the money. In this case, there was literally no one around to ask, so we had a momentary jointly-felt twinge, and then kept the money. It wasn't in a wallet or anything.