I recently attended a "course" and the instructor told us that when testing the Class II insulation test on an appliance with no metalwork, to hold the probe against an arbitrary part of the plastic enclosure and do the test like that.
To me, this seems like a pointless exercise. The chances of placing the probe over a part of the case that is failing invisibly seems remote. (If the case were cracked, you'd see it during inspection.)
I can fully understand probing any and all pieces of metal and screws (If present) but can't see why probing a random point on the plastic case would be a satisfactory test. It wouldn't be repeatable in any event but can't see what it hopes to achieve.