Your dog got tired, fell asleep and something got them? I don’t know what predators you have. I put a lock on mine. I have actually had raccoons easily open clips and stuff.
I once lost a couple of hens. Looked everywhere. Large preditor birds are prevalent in my area being so close to the river so I have noticed that if one or two of my chickens get separated from flock while smelling the flowers, lol if that large bird comes over they find the nearest bunker and they actually hide for 2-3 days before they come out! So, keep looking. I finally gave up and the 3 night, I go out to coop and both sitting there looking at me like, “So sorry!, and its a scary world out there” look. So don’t give up just yet. They will roost in trees as well. But think what is their color around me. That may be where they are hiding.