I found this on the Boeing 777 and it was on the Captain's side behind the speaker.
1 Answer
That is the sensor for the “cockpit door decompression-sensing latching system”. Most pilots I know are not aware of this system as it is not usually covered during training.
I have seen this system tested by using your mouth to blow a stream of air over the sensor. This creates a brief pressure differential which automatically unlocks the flight deck door.
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$\begingroup$ Very interesting! Does it actually open the entire flight deck door? My FCOM says something about "two hinged decompression panels". It sounds like only these would open. $\endgroup$ Jun 19, 2022 at 17:56
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$\begingroup$ Yes, it unlocks the door. The decompression panels are separate items. $\endgroup$ Jun 19, 2022 at 19:02