Well two things come to mind. The first thing is that someone used the old coat hanger (or packing tape) trick which involves pushing it down inside the door window and manually tugging up the lock mechanism. I have used this in the past when I have locked my keys in the car, and it normally leaves some kind of small damage or marks on the rubber.
The second is that someone has a bit of high tech kit that monitors car remote locking frequencies, copies the codes it receives and then is able to repeat them to unlock cars. Expensive but common in other parts of the world.
I guess the third is that the car was mistakenly left unlocked, and the fourth is that there is something supernatural occurring.