Secrets in the Walls

We recently replaced the medicine cabinet in our bathroom (while cursing manufacturers of cheap fiberboard accessories that begin disintegrating after your first shower). The kids decided to hang some artwork in the wall cavity behind the new cabinet, hoping that someone will find them in “three hundred years.”

The secret cavity
Close-ups. One each by Griffin, Maggie, and Oliver. 
The cabinet installed
Who could know that behind these mirrors lies a cache of secret art?