10'C workers reluctant to fly
5.5'C bees become incapacitated
4.4'C bees die if isolated / outside of cluster
Either your thermo temp and the apparent temp at the hives was different or you have some bees who like living on the edge!
They could have been short cleansing flights (orientation flights? unlikely to have much viable brood left after the -10'C cold snap) and bees can generate heat. So the question becomes "how long does it take for a flying bee to reduce core temperature from 30'C to 3'C and die under ambient conditions of 3'C?"
Both my hives had a few flying bees today. 6C on the "bee thermometer" next to the apiary. I think this is about their limit when it comes to cold. They've had a big kicking-out session of old bits of bee, dead bees, a couple of propolised moths, a harlequin ladybird (alive but very unhappy - stung?) and some kind of fuzzy stuff which may once have been a larger moth of some sort.
There were a couple of dead bees blocking part of the mouseguard. Being a slide-in type of guard, I removed it for a while and scooped out the obstruction. Several guard bees gave my gloves what-for whilst I did this, so it was obviously warm enough outside for them to get aeriated. Later in the afternoon the temperature dropped again and it started to sleet... so I guess that will be the girls' last outing for a while. It is amazing though how they keep the place tidy even during a bad winter.
I went into the hives yesterday as the sun was shining- I did oxalic treatment to all- but at least half the colonies were so low down I thought I had lost them...They are really tucked in for the long haul and I have had none out flying this week- and its getting colder...