Had a new one back in November on DFW-LHR.

About 30 minutes before landing after the Bose headsets had been collected, the purser made a polite, non-accusatory announcement along the lines of "we are still missing one Bose headset. Please double check your seat and help us locate it."

Apparently it was not located, because the next announcement was soon to follow. It was still polite, but a bit more firm. She essentially said that the headset was still missing and that all items must be accounted for before pax can deplane (I forget the exact term she used in reference to inventory being complete before pax can deplane).

Another few minutes passed, and FAs were making continuous passes up and down the aisle. I had my own Bose headset and the FA asked to quickly take a glance at it to verify.

Finally, a much more stern announcement was made that essentially told us that we'd all be sitting on the tarmac until the missing headset was found. I'm guessing it was turned in because there were no more announcements and we deplaned without event at LHR.

Does AA have a SOP for when headsets go missing? This was the first time in quite a few international flights that I've ever witnessed it, and it was on a 77W so it's not like someone could take the headset and plug it into their iPhone.