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?
Legally and technically they are not yours to play, but common sense says, that it wouldn't be practical to leave those coins in. If everyone thought that would be a way to go, no one could even play that machine with their own coins before those left in coins are used. And in reality there's no one in the world going to make you pay back those coins. It's just a minor morality issue where's no right answer. I myself have played those coins multiple times, because no one leaves even $5 in. It's always in cents.
And if you are feeling guilty, you can always leave that amount of credits in after you stop playing.