All the bees sitting absolutely still and all facing upwards on the comb.
Yup, exactly what they do!
Ok, this is interesting. I saw this behavior in August, in the middle of the dearth. It was a small colony, maybe 3 or 4 frames. They had just about run out of food. It seems they had stopped trying to forage and were trying to conserve energy. All lined up in rows with heads pointed skyward on the comb. They were one or two bees deep, not a real cluster. It was very distinctive behavior and I've only seen it that one time.
I'd love to hear a detailed description of anything you remember about how your bees acted. From what you said, it sounds like they were low on food too.