I don't know why I decided to look since I knew it was in there, but it's a good idea I did.
My passport isn't in its usual location. I can only assume that I failed to put it back there after returning from UDS last November.
Turned the room upside down. No sign yet.
At least the passport office do a one week replacement service, which is a "bargain" at £112.50 (plus a trip to Peterborough).
Where else should I look?