Under the Mattress

Last night, I went in to check on Middle Son a little before bedtime.  He was lying underneath his mattress.  I don't think I've every seen him do that before.  Oldest Son was in his room playing his keyboard.  His keyboard is right on the other side of the wall from Middle's bed.  I thought perhaps Middle was hiding from the noise, so I asked him to turn it down.

I mentioned it to my nanny today.  She said Middle had done the same thing to her this afternoon.

I wonder what this is about.