So I have just returned from a two day trip to find that I have left the lid off my Lasius niger colony. Most of the workers and all the brood are missing however the queen has stayed behind with about 15 workers. The queen doesn't look like she is about to leave and I can't see any ants returning.
I'm baffled as I do not know if they have moved or have taken all the brood to a warmer place to heat them up.
They may well be in the process of moving the nest which you have interrupted. The escapees will know where to go back to but won't be able to get back in. I'd suggest you leave some honey out for them and try trap as many as possible. It's possible that the brood will now be lost unless you find where they have moved it too.
They are stored in a shed cluttered with nests, it's no easy task following them. I have absolutely no idea where they have gone to so I have left the lid off. If they come back than great but if they decide to leave then I wish them the best of luck. Absolutely gutted though, was close to 1000 ants.