Little symbols indicate stations/AP-networks/frequencies. Stations and AP have MAC addresses, frequencies (channels) have not.
How can they be there if no stations or AP are visible?
Either their signal cannot be decoded (too weak to decode, to much distorted, non 802.11 protocol, just noise)
Non 802.11 are things like bluetooth, remote controls, wireless keyboard & mouse, DECT telephone, some camera systems, sensors, home automation, …
If you enable “Receive Errors” in the Settings of Snooper, you might even see more.
Instead of “all”, you can filter the type as “frequencies, networks and stations” in Snooper.
In the last column, snooper counted many “stations”, but could not give a 802.11 MAC address.
Channel 13 was very busy.
Your last scan is rather clean. OK channel 6 is busy, but will be no problem. A really clean 2.4 GHz is very rare to non-existing.
You have the tools now to find a clean channel for your AP.