Grounding and why charge leakage occurs
It is not unusual for the mains supply to be deliberately connected to ground via small high voltage capacitors, to reduce emitted radio interference. These capacitors are rated to withstand high voltages safely, and to "fail safe" (i.e. not form short circuits in the event of an accident or excessive temperature.) They are usually identified as "Class Y" or "Class X2" on their case markings, typically 0.1 uF 275V or 400V.

These will conduct a small AC current to a metal case, and if the metal case is NOT correctly earthed, it is possible to get a mild shock from this current, but it should not be dangerous.

I have also measured about 110V AC on exposed metalwork simply from the capacitance within a (230V) mains transformer (the short circuit current was only 30 microamps but the "tingle" could be felt)

I  would however agree that any other source of leakage from AC mains to metalwork should be investigated - the dangerous ones would usually show up with DC resistance measurements, unlike the above.