This is a UK install for reference.
Long story short is I essentially have a 13A plug wired direct to an RCBO. Output of the RCBO is connected to a 13A trailing socket.
There's a multimeter inline between the earth of the 13A socket and plug. Functional earth is connected to the earth pin of the 13A plug.
With the RCBO off, as expected, the current on earth is 0A. Turning the RCBO on causes a current of 6mA to appear on the earth.
Why is that?