If the cars parked on the drive at night than locked away in the garage then an external security cam is a must, first thing I did when I got the 140i. I don't think a dash cam would be much use as you ideally need something watching the car and surroundings. Unsure it's a great idea keeping spare house keys in the car even if they won't know which house they're for. I would certainly be on your guard as they might have been testing your security and will take the car next time, hopefully just an opportunist.
Tim.
Tim.