He was furious. He was sure it was the boys and I was sure it wasn't. They are sneaky and can be destructive BUT they could never keep this a secret! (FYI - this is the wall between the guest/craft room and the back room in the basement). So he called the boys downstairs. They were shocked. They thought someone broke into our house and did this. Simon was convinced that someone came in with a chisel and "chisel, chisel, chiselled" it.
Here is our culprit. I think she got herself locked in the backroom and found a way out - of her own means. Now we have one more "thing" to add to the to-do list.