I sure that some of our more technical-oriented members can give you a better answer than I, But I just wanted to mention an experience I had once.
I was trying to figure out which breaker was for the sales counter at a store once. I could see the cable for it's entire length back to the panel and individual breaker. There were maybe 4 receptacles on the circuit and the only items plugged in were 1 computer through a small UPS and a Calculator. The computer was off, the calculator was on and I could see it from the panel location. I could not get that calculator to turn off even after removing the wire from the breaker. With the circuit off (and wire disconnected) the calculator was still on and I was reading about 115V to N/Gr on the wire.
What seemed to be happening was that the UPS was feeding back into it's supply circuit somehow and into the calculator across the room (same circuit). I could see nothing wrong with the wiring or any signs of tampering with the UPS either. I took the UPS out and everything was normal again.