I allow neighbors' kids to use the nest. I called both neighbors. One who was home suggested looking outside the nest. I did. The kids had dragged the mattress outside behind the nest. Now it is stained, muddy, soaked. I called the neighbors back, got the mom of one who was very upset and offered to pay for a new mattress (this one was custom made). I told her no. Haven't heard from other neighbor who has two girls. All are 10-12 which seems old enough to respect others' property. I have put the nest off limits, vacuumed the mattress and put it in greenhouse with fans - hopefully it will dry out and not mildew. don't know how long it has been out. We have had big rains for days on end here.
Martha and Mom appeared last night to apologize. It was Martha with a school friend. Other neighbor's kids were at Bible Camp and innocent.
At least I wasn't having a senior moment. Overall it is funny. I knew I took it, but since it wasn't there I kept looking for it anywhere and everywhere.
So the lesson is believe in yourself. And the lesson the kids learned, respect others' property or you may not get to use it.