My guess is also that the other 3 was unlocked. This would be because his keyfob, like many others, does not work at all. In other words, you are one of the lucky ones.
For him to lock and unlock his 3, he has to do it manually, a bit of a nuisance.
This brings up another question. Do you know how to manually lock and unlock your 3 and more importantly, if you do, does the unlock function work?
Since the mechanism is mechanical and used very little, it can freeze up - well maybe it locks, but does not unlock.
At least once a year, I do the lock and unlock thing to test it out. I might add that I always roll my drivers door window down first - that suggests my level of confidence so to speak. So far after thirteen years, all good including the key fobs, but there is always tomorrow.