The surveillance system on the third floor of the unit showed a round light group flying around at around 2 a.m. What is it?

It was flying dust, lint, mosquitoes, etc.

Because the light at night is reduced, the camera automatically turns on infrared lighting. Any object will reflect infrared light back to the camera lens, so you can see a lot of things flying around on the image. This is normal.

Obviously there is something like a bug on the camera. It was too close to be imaged, and coupled with the direct reflection of light, when the insect moved it looked like a moving ball of light.

These are all natural phenomena. When a camera shoots a computer, the computer screen can appear to be constantly refreshing, which is invisible to the human eye. When we first seized power after liberation, when surveillance cameras were installed at the border or in the mountains, a bug similar to a grub would appear rapidly in front of the camera, forming streaks of light.