I'm no lawyer, but seems to me there ain't really any laws yet dealing with newfangled digital coins like cryptocurrencies specifically.  But we can take a look at the legal framework around regular physical stuff people find and draw some parallels maybe. 

Way I understand it, whether you get to keep something you find depends on the details,  like how and where did you find it, what is it exactly and whats legal in your jurisdiction.   

General rule of thumb is if whatever it was got abandoned - as in, the owner done gave up on getting it back and said sayonara - then finder's keepers.  You find it, it's yours now.

Few exceptions though.  Say it's property whats been lost, just misplaced accidental-like and the owner still aims to come back for it.  Or it's stolen goods that got taken without permission.  Then finding it don't entitle you to squat.

So in the case of cryptocurrencies it'd come down to the specifics I reckon.  If theyre truly abandoned with no chance the owner tries tracking them down again, you could argue they're legally yours now.  But if they're lost or stolen, probably not.  Of course, that's just my casual understanding.  Like I said, aint no lawyer here.