Aha, it is found! Apparently it slipped down between the bed and the wall last night, at the same time my phone and its backup batter did, and I overlooked it when I fished up the phone and battery.
This brings to mind the rumored pirate practice of burying a less valuable chest, and/or a scary skeleton, in the ground a bit above the real treasure chest, so that anyone digging downward there will encounter the decoy chest / skeleton and stop digging, thereby overlooking the real treasure.