Yes, those are the sensors.
One last thing, was your person in the car absolutely still for a while after you locked it before they moved around to make the alarm go ?
Im just wondering if maybe the system detects movement within a few seconds of locking it deactivates the sensors ?
Also, were the windows closed when doing the test ?
BMW if you are reading, I need a job, Please. (Really, I do. Unemployed. 2016)