I left it on the kitchen counter while I fed them, then got busy with other stuff.
After M. was working at the same counter, I asked her if she had seen it.
No, she had not. It was nowhere to be found.
The obvious suspect was lying on a rug by the front door: Fisher — Raider of Kitchen Counters, Eater of Everything.
Going from memory, I checked the mushroom book, and it looked like I had had Boletus chrysenteron, which is edible. (From Fisher's viewpoint, if it fits in his mouth, it is edible.)
Mushroom taster, that can be his new job.