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.