Whilst walking home this evening from Asda, I found a £20 note lying on the pavement. There was no one around and nothing to indicate to whom it belonged. I picked it up and put it in my pocket and it's still there now. My mother called me a "jammy sod" and my father said, "Oh well, probably some poor old lady's."
This is one of those questions which comes up from time to time, and with different circumstances. Nobody around, no apparent "loser" - I'd have done what you did.
It can't have fallen out of a purse or a wallet, so must have fallen out of someone's pocket. There is a cash point there outside Bargain Booze, but no telling how long the note had been lying there on the pavement, so not likely I could trace its rightful owner.