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?
I've never heard about that rule and I also don't have any personal experience with that, but as far as I can understand, there shouldn't be any problems if the credits left in the game aren't a considerable amount like maybe a few cents that can't even be used for a few spins, no one might even notice if you add your money and start playing. But just to be on the safe side, maybe it's advisable to ask an employee before adding your money so that they can do something about it.
Asking an employee about the left credits is better in case the person who left those credits comes crawling back after a few drinks and is not in his senses and says that you have used his credits and whatever you have in that game is his, though I'm not sure how casinos react to such things because one can clearly be lying as well but it's just better if you confirm it from the management before going in.