W211 rear interior light unit: alarm sensor?

[selphy]

Today I tried the procedure to hold the keyfob lock & unlock buttons down until the keyfob LED flashes and then the keyfob only opens the drivers door unless you press unlock a second time. Useful feature if you're in a rough neighbourhood. Repeating the procedure toggles it back to the normal mode where all doors open.

That's all good but I noticed when I did this from inside the car, a small motorised flap in the rear interior lamp unit dropped down. Unlocking or locking the car again made it go back up. I've never seen that thing before. What is it? it looks like an ultrasonic transducer, perhaps for the alarm?

 

[Londonscottish]

I've been wondering the same thing.

I once locked the doors when my kids were waiting for me in the car. The alarm went off and that thing descended from the roof.


So something alarm-related

 

[KillerHERTZ]

After 10 secs or so, the motorised flap always comes down regardless, as you say its the ultrasonic sensor.