I gave Middle a bath last night. After the bath he still smelled. I couldn't figure out why. When we went into the living room I sniffed him again, and he smelled fine.
This morning after taking a shower, I grabbed a towel to dry off. I discovered that the towel was crusted with poop. Apparently Middle wiped his rear on the towel last night. Now I know why he smelled bad in the bathroom but fine in the living room.
The smell was coming from the towel instead of from him.