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?
To avoid getting to much pressure regarding on this thoughts much better if you don't use it since who know that people just forget the balance left on the machine. So to avoid being misinterpreted by other people just call the casino worker and told them about the situation so maybe they can reset the machine and you can start playing with your own clean balance. Although that's other peoples negligence but we shouldn't take advantage on that so we can play with clean conscience.
Some other people take advantage with that but don't follow what other do especially if you are the type of person which cannot take any hurtful comments come from other people since for sure you might get criticize for taking advantage on such situation.