It'd be a tough call, but after losing my actual wallet multiple times and having it returned (with the cash still there), I'd try to find the rightful owner. I couldn't imagine losing 50btc, and wouldnt feel right taking it. If I legitimately couldn't find out whose they were, then I guess I'd HOLD!
I left my wallet on top of my car once at the end of a construction job and it was the kind of area that developers where pushing the crackheads along. Police found a girl tied up in one of the basesments,hookers would be seen wearing any clothes we left on the site,they ran power cords from our site 4 houses over so they had power and stuff like that.
Anyways I get to the gas station and no wallet. My boss calls me to let me know some guy picked up my wallet and gave me his number.
Meet up with the guy and he had called every business card in my wallet to track me down,said people where just driving over it laying dead in the middle of the street.
Job was cash at the time as well,so I had a full pay cheque sitting in my wallet.
Bought the guys family dinner,just could not believe there are people out there with such great standards in life.
When I was a kid I was like 11 years old. I had just finished working on a renovation for a family friend who paid me $20 a day!
Anyways,I was sitting next to this guy that was asleep the whole trip back home on the bus. Get home and the phone rings. This sleeping passenger had found my wallet
sitting next to him when he woke up with really only $20 and my student id. He phoned every one in the phone book with that last name looking for me to return the wallet.
Would not take my money either. I mean I wish I could be as awesome as these people where to me.
Something to ponder I guess.