I measured when idle. about 35C° it can go to 40C° under a lot of traffic.
Compared to the U7 Pro (50-60C° actually too hot for me to hold it) UCG-Fiber is pretty cold

The Unifi has passive cooling and no audible noise.

It gets moderately warm and should certainly not be in a closed box.

Regardless of what others have written here, the UGC Fibre has a fan. However, it is hardly ever audible during operation without an SSD. There are ventilation slots on the side that must not be covered. The device gets warm to the touch during operation (you can touch it with your hand).

If you install an SSD or buy one of the variants with an SSD installed, the fan will be clearly audible, as it then has to remove the waste heat from the SSD from the housing.

I can't hear a fan. It does get warm, but no warmer than my Zyxel switch. So not noticeably warm.

The device has no active cooling, only ventilation slots. In my case, the device already gets warm, so it should not be packed and air circulation should be ensured.