Even if the keys are in the vehicle it could be the result of lost keys, key fishing thru letterbox etc.
If keys were just left in the vehicle then it could get messy, hopefully though if someone had the key it was the result of one of the above..
A friend many years ago broke into a neighbours repossessed house as they left keys on the kitchen worktop & they were fished out thru the catflap. No harm done but point is it happens