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 give an example from the physical casino machines, in my country the card system is used in all slot games and it is possible to deposit money directly with the machine. For example, when I deposit money to my card from the first machine and I start gambling with this is reflected in my balance and I start gambling. Afterwards, I log-out of my account again with my card for the amount I want and I can log-in with my card again on a different machine and continue playing with my balance there. Although this is of course not a valid method in every physical casino but I wanted to point this out based on my own experience and observations.
Additionally, once you start gambling and exit the machine with your card, data such as the current account balance and the last game amount disappear from the screen. In this way, with the card system various games can be played with a one-time deposit without leaving any balance in any machine and your balance last game winnings or last game bet amount are not displayed by the next person.
For this reason, it isn't possible to use anyone else's balance or voucher in physical casinos in my country. Finally, to answer the question about using someone else's voucher coupon, I think it shouldn't be used because it would make you legally guilty if this situation is detected and I don't find it ethical. There is not much difference between committing theft by stealing money from a person's pocket and using a voucher code belonging to another person.