One of mine did the same thing today except I caught them before they got too far. The number of bees were way down, very little brood (capped and uncapped) Had honey stores, a few small hive beetle but didn't see any larval stage. They just left. I was able to put them in a 5 frame nuc with frames of honey and pollen, so I'll just wait and see what happens. I started the mite treatment yesterday but just peeked in, not a thorough inspection. I noticed the lack of brood but thought it might be a queen issue.