Why does the instrument measure a current level with the current probe turned off?

For example, if you have Metraflex 3003 XBL current probes connected to a Mavowatt 240 (set to 3000A) and your current probes are turned off, the Mavowatt may still measure a current equal to 35A.

This is internal noise and has no significance.

For reliable values, the Metraflex probes must be switched on and the current flow must not be less than 10% of the maximum value (i.e. 300A for 3000A).