Is it possible they have a nest outside the hen coop? Quite often one lays there a few days, and then another discovers it, and starts ......last weekend I spotted a hen going into a builders sack we have, full of haylage......I had a look and there was 6 eggs!! Some of our hens lay in the barn, and all sorts of peculiar places, and quite often its only because I see them coming back that I discover their secret place!!