Noticed before when swapping the Jaycar "crappy" power meter to different sockets that the *plug* of the UPS gave me a bit of a buzz when it was chirping away in backup/no mains mode.
Obviously it's not going to be backfeeding its full inverter power into the socket that it's plugged into, otherwise it would be attempting to power everything (other devices on the power board, the rest of the house wiring, ...)
Just curious what it may be, whether it's common or a cause for concern.