Hens are sneaky like that!
Actually they have a natural instinct to seek a safe place for there offspring and eggs in a nest to the other hens mean( this is a safe place!)
If you dont already have fake or decoy eggs in your nest boxes I recommend putting some in. No need to spend a lot of money...
I think I would look around your yard area.
They may gave found a different spot to lay, like under a bush.
If there is no molting, and no broodiness and their combs are red. They are laying somewhere