After a few warm days of observing the candy board supers on the hives, I started to suspect robbing in one (the one to the left). This one has Honey-Bee-Healthy in the candy, which might be the main tip-off to other bees that there are goodies to be had. The other hive, where I decided not to use the HBH, has much less bees hovering around it.
The main point of attack, if that is what it was, was the rather large hole I put into the super, right at the level of the candy, for ventilation and winter access. Luckily I kept the two blocks I had chiseled out of the super, and today I put those back into those holes.
Ventilation and top access are not an issue yet, anyway. Also, the bees should be all set with the small hole in the entrance reducer at the bottom, since there is not much to forage, except for water – anywhere there is a puddle or some standing water in a bucket, the bees are there.
Their castles should be eminently defensible now.