In The UK
..............and I may be / am wrong.......
There was many moons ago about the late 80's/early 90's, as published in the supliments, the phrase 'All stations this frequency, stop transmitting X3, MAYDAY in progress' I believed this to be related to 121.5. Such call could be made by any ground or air station privy? Have I remembered right?
That would kill the practice pan, all chatter, though there seemed none then, yet being a costal Ops guy I monitored 121.5 on the GTA on scan.