I probably would have asked her if she'd just made a withdrawal. If she said yes, I'd say I found money left in the machine and if she new how much was there .. I'd give it to her.

If she said no, or didn't have the receipt ... I would have taken it to the bank (if the machine was attached) ... or to the local police station where it could be claimed. I'd leave a note by the machine tellling "to whom it may concern ... if they accidently left cash, take their receipt to whereever I left it."