Must be a cultural thing. Here in the northeast USA, I've never heard of any such expectation.
I do have another expectation, though: that the wallet most likely won't be returned at all, or if it is, that 100% of the cash would have somehow gone missing by that time.
I guess if someone actually came to my door with my wallet, I might be sufficiently surprised and grateful that they made the effort of coming to me that I'd give them some of it. :)