I've seen this a few times as well. Usually with ungrounded older units and some with unpolarized plugs. Could be a compressor shorting to ground, a chafed wire touching the frame, a neutral to ground connection with the unpolarized plug reversed. This is why I always carry the ticker. Bring your ticker to hotels that you may visit and see how many hvac units as well as desk or floor lamps that are live! Scary stuff, especially since many of these hvac wall units (ptacs) are running at 277 volts. Anybody who's ever worked in a hotel knows why, $7 an hour maint. personnel repairing things they don't understand.
