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?
First time I am hearing about this rule, maybe it's something country specific and varies from place to place. When I was younger, I always liked to check all the slot machines if someone maybe didn't use all his money in the game, or forgot to take all their winning out of the machines. It's like checking the locker rooms in the swimming pool if people forget to take their 1 Euros out of the lock. It happened quite a lot that I found some money and I directly used for my own game to try and make a profit of it. So far, I never ran into any trouble and people didn't come back to look for their money or wanted to play again on the same machine. In a video I saw that in Japan people are much crazier about slot games and are going to queue hours before the opening of the casino only to get their seats at their favourite machine. In such a place I am sure people will reserve their machine if they are going to the toilet or get some food. As for the legal aspect I think this different from country to country, but since you don't know who lost the money it would be hard to give it back. Just giving the money to the casino would also seems not right, better to enjoy the free round.