I'd be concerned/creeped out by it and wouldn't want to have anything to do with it. That's not a "normal" thing. It's not even like someone hiding cash, say, under a mattress at home. Aside from serious "iffy-ness" (crime) that could be involved, I'd be thinking about things like a kid taking cash from someone at home or even someone with Alzheimer's leaving cash somewhere (maybe not being the one to hide it but leaving it somewhere, having someone else find it/take it, and having that person be the one to hide it. Years ago I saw a man who had Alzheimer's walk to a nearby store and dump a pile of bills on the counter. The kids who worked there were really uncomfortable about it and helped gather up the money and tell the man he didn't need that much. They apparently called the police, who apparently knew him and they (I guess) gave him a ride home.
In any case, I wouldn't want to keep it. I'd contact police (and if someone there kept it, well, let them -because there's also a good chance that someone called in a report about a theft.).
Sorry... that's just too weird/peculiar a thing to want to have anything at all to do with it.