You might be over-thinking this. It's possible to disable the factory alarm motion sensor by double-pressing the lock button on the key fob. This is a feature for people (like me) who take their car on ferries, and who don't enjoy listening to a cacophony of car alarms while on the ferry (I appear to be in minority here, if recent trips were anything to go by)

Apparently a fair few owners don't know this feature exists (didn't read the manual/didn't have a manual) and were in the habit of "checking it was locked".

So, if you can't remember if you locked a BMW, UNLOCK it first, then LOCK it. Locking twice disables the alarm.