I got a Tetramorium caespitum queen last year and before hibernation she had about a dozen workers and some eggs etc. After hibernation I checked on the colony and everything seemed fine. They are living in a small glass tank with some loam, they have chosen a lump of loam which has gone very hard, almost like a stone, and dug into it. I have never seen these ants out foraging, I have only seen them when I lifted the stone to check they were still alive. I have never seen any evidence that they are taking food, but with the amount of ants in the colony - they must be. So I usually just check their food and water and leave them be.
Today I put half a grasshopper and half a pea in the tank, then waited 20 mins and went back to check - the queen was out foraging. I'm pretty sure it was the queen, the workers are very small and they don't have majors right?
Anyway, I see no reason for her to be out, there havent been any dead workers dumped outside of the nest, I checked the stone they live in and I don't see anything under there anymore, no eggs, brood or workers. just the queen out by herself wandering around. Did she eat her whole colony?