That's an interesting question. I think it's safe to say that
there are people who use these machines who sometimes don't really
pay attention to the screen, other than when they're actually
inputting their bets. Let's say John Doe walks up to a machine
right after another bettor walks away from it, there's a balance
on the machine, and he doesn't notice it. John inserts his own
cash voucher, and now the machine displays the new total balance,
which is John's old balance plus the amount which had been left
in the machine by the prior bettor. John looks at the balance,
and thinks, "That looks like more than I thought I had.....I
guess I won more on that last game than I thought." He makes
his bets for the next game, takes the voucher for his balance,
and cashes it. Is John a criminal?
I don't know the 'legal' answer to that. My opinion, though, is that each bettor should be responsible for taking his or her balance out of the machine at the end of each transaction, the vouchers should be bearer instruments, and possession of a voucher should give the holder legal possession of its value unless it can be conclusively proven that the voucher was stolen. (And by 'stolen', I mean taken from its rightful owner without his/her permission, not simply removed from a machine where it had been left or picked up from the floor.) Bettors are responsible for counting their money at the teller window before they leave the window. They should be just as responsible for their money at the machines.
It would be different if the machine transactions were PIN based. Then, the rightful owner of an abandoned balance could be accurately determined. As it is, though, there's no way of proving who left the money in the machine, so, IMHO, the vouchers should be bearer instruments.