Well, last night he was playing with it, tossing it up in the air and pouncing on it. And then he LOST it!
We looked everywhere. He was making little chirping sounds like he makes when he loses his real live mice under the sofa. I moved stuff around. Looked under stuff and on top of stuff. I even looked in my sneakers. At last we found it. It was hiding in a corner behind the sofa.
Whew, it nearly got away.