Who and what?

If you are operating close to controlled airspace and not requiring an Air Traffic Control (ATC) service you are always encouraged to monitor the relevant ATC frequency.

Rather than squawking 7000, you are encouraged to select a local Frequency Monitoring Code (FMC) or listening squawk of the nearest appropriate ATC Unit. You should already have Mode C selected, to indicate that you are monitoring (listening to) that unit’s frequency.

https://airspacesafety.com/listening-squawks/

http://www.caa.co.uk/News/Listening-Squawks-update-for-GA-flying-season/

Exemples

UK: https://airspacesafety.com/wp-content/uploads/2019/04/S01_Squawk_A5_Leaflet_April_2019.pdf

Et

https://airspacesafety.com/wp-content/uploads/2019/04/FMCAreasAeroInfo15FEB19.pdf

Italia: http://aeronauticalinformation.it/index.php/2018/05/30/fmc-frequency-monitoring-codes/