The general models of cars on hire are alarmingly simple to break into. I spent many years in the breakdown/recovery service and you get a lot of calls to "lock outs", where people have locked themselves out of their own cars. apart from very modern expensive imports, most cars are quite easy to get into. If it is a top range BMW or similar, breaking the glass with a half brick takes a thief about a milisecond to get in. The general rule is never leave anything of value in a car, no matter if it's only for a minute or so. Someone else having a spare key is a bit dubious as well - do they follow you wherever you go on the off chance you'll park up and leave money in the car - seems pretty unlikely. Sorry for your loss, and I hope you still had a lovely time and won't let it stop you returning.