What is your body language when you find the items?
My first theory is that the dog's are reading your body language. If you are upset about this then they will know and act sorry.

What about when you get home and the dog is hiding and you don't know what's wrong?

My theory for that is that they've gotten in trouble for a mess before (ie a person came home saw a mess and then said bad dog or whatever). So the dog associates the mess with getting in trouble, BUT the thing is that the dog doesn't associate the actual act of making the mess with getting in trouble.

I don't know if that makes any sense to anyone else but me. LOL But that's what I choose to believe.