I have had this happen to me on a few occasions as a child, once at Kindy, my parents were late (afternoon kindy) by about an hour and the teacher stayed to look after me, 'forgot' and started to lock up but then realised! We were going away and my parents made it about 15 mins away before realizing I wasn't in the car! And my grandpa accidentally left me at the supermarket twice.
Glad she's ok because I had nightmares as a kid from my experiences! I would be very upset and angry!