Don't do anything. As others have pointed out, it is just like stealing someone's wallet, even if they weren't too smart to begin with and picked a weak passphrase.
If you found a wallet in a taxi cab, would you keep it or try to find the owner and return it? I guess it depends on your morals. I would return it, but that's just me.
If you found a wallet in a taxi cab, would you keep it or try to find the owner and return it? I guess it depends on your morals. I would return it, but that's just me.
This is a great analogy and if you think about the situation it points out the dilemma that the OP is facing.
Doing nothing is equivalent to leaving the wallet you found alone. So, you find a wallet in a taxi, and you just leave it there for the next guy to steal. If the OP does nothing with these bitcoins, then he is just leaving them there for someone else to discover and steal.
Transferring the bitcoins to his own wallet but not spending them is equivalent to picking up the wallet, but not spending the money that is inside. He can then make sure that nobody else steals it, but he's left with the complicated project of trying to find the original owner. If that wallet in the taxi has only cash and no identification in it, how would you prove that you are the owner of the wallet?
You say you, "try to find the owner and return it". Ok fine. How long do you look for the owner of that wallet in the back of the taxi? A few days? A few months? A few years? If you still haven't found the owner after 50 years, what do you do with the cash that is in it? Do you just toss the wallet and all the cash into the trash?
Good point. I guess how long you try depends a little on common sense and your own level of perseverance. I imagine after a reasonable effort has been made to find the owner, without success, then it would be ok to keep the BTC, or, even better, donate them to some charity.
. If you can guess someones passphrase imagine what will happen if everyone starts doing the same thing. That means its very unsafe 
