There is a general rule in land based casinos that a gambler shouldn't take credits left on a slot machine by someone else. Basically, from what I understand, if there is still credit showing on a slot machine. Don't cash out or add to your money you put into the machine. Right? The question is if you find a cash out voucher resting on the machine itself or on the seat, can you legally use it?
Usually when that is the case it's only like that when a gamble leaves to take a piss or to do something else, so the most sensible thing to do always is to avoid that slot machine and perhaps wait for your turn in cases where no slot machines are available for you to play with. Just apply that logic to any other game or table setting and it would all make sense. For example, you found an empty seat in a poker table, you'd try to play with the ones already at the table but you suddenly noticed there were chips on the seat's side of the table, would you, as a functioning member of the society, take that as an opportunity to play on someone's dime, knowing that there's the possibility of him/her getting back to finish his game? Or are you going to be sensible and wait until everyone's finished?