Solved

Alright, the mysterious diaper disappearance is no longer a quandary. A friend of mine (a mommy, of course) brought up a point that I had not considered. Had Phoebe worn a diaper to bed at all?

"Of course," I thought. "I wouldn't forget?...Would I?"

Ah-Ha! Suddenly I remembered that I had taken Pilate's that night, leaving Xee in charge of getting Phoebe ready for bed. He got her into her jammies early and was going to wait until bedtime to put the diaper on...

Then I got home. Xee forgot about the diaper. So off to bed she went...and you know the rest. I did a load of soiled sheets the next morning, but at least we aren't waiting around for a smelly diaper to turn up. Well, another parenting first. Another good laugh. Mystery solved.