OK, time for an answer.
I know the numbers are a bit hard to see, but if you look at the scale on the left hand side, the horizontal lines are one kilo apart. That blip on the curve is probably a bit less than one kilo.
Let's think about the mean bees now. Each bee weighs about 0.1g (range 80 to 150 micrograms, according to my text books ). So if we had the weight increase resulting from mean bees, that would be around 10,000 bees. The mean bees were a new nucleus, only about 3 weeks after installation. I would think the whole colony has less than 20,000 bees in it, and it is pretty unlikely that half of the colony left the hive in the dark and landed on the adjacent Arnia hive! That would be a swarm-sized mass of bees, or about 2/3 of the size of a package. I thought it was bees too, until I did the calculations, and realized how unlikely that was.
Then I remembered. I was working with my naughty husband, who is a bit like a 3 year old sometimes. He was in charge of the smoker, while I tried to position the hardware cloth over the mean hive entrance and fix it down securely in position. When I turned my back, he balanced the hot smoker on top of the sloping roof of our Flow hive. I didn't notice it for a couple of minutes, and during that time, the Arnia scale captured evidence of his deed. We have since discussed good places to rest a hot smoker.
So the closest answer was @Webclan, although there is nowhere on the hive to "hang" the smoker. Thanks for the fun, all!