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?
You could use it if no one came back for it after an hour or so, although you will have to stay that long for what could be something so trivial of an amount. I say an hour because there are times wherein players still come back to their machines after a smoke or coffee break, or to eat something then play again. If no one comes back, just take it. Who cares about it anyway? It's as good as being lost if someone forgets it. Also, casinos wouldn't really hold you at gunpoint should you come and get those vouchers. As for the credit, it'll just be your conscience that you'll be against, so you decide whether you'll get those credits from the machines or not.
It varies depending on the credit amount left on the slot whether you'll decide to use it or not as most of the time, people will leave their slots especially regular players and advise casino staffs before leaving to avoid these from happening. Also, there are instanses where someone used these slot machines that had these credits inside and was advice to pay the amount they've played especially if the casino staffs was informed. I'm not sure about other casinos but these are what I've personally saw while gambling.
Also, it's quiet common to see some slots that has some credit left on it which isn't a lot where people can just use it.
It would be vile for the casino staff to not prevent you from using the machine while knowing that there is still someone using those with their own credits. It's scheming if they let you play in it and they see you without informing you first. Anyway, this has always been what I observe in casinos. No one really leaves their machines with balances without informing the closest staff. It's as good as lost when you leave with your credits for too long - at least from where I'm at.