Its hard to determine with certainty if your voltaic is giving you a false positive. The only way to actually check wires and be certain they are dead is with a proper volt meter that you put on bare conductors.
Kind of an annoying answer since you don't have one i understand.
I am pretty sure you had all the breakers in the main house panel off at the same time? In which case thats basically the same as the main breaker being off, when you tested for power on this circuit. If thats so than I agree with what the last poster said.
If however you flicked each breaker off one at a time, tested it AND TURNED IT BACK ON to proceed with testing the next breaker and the circuit never seemed to be dead, than it could be something else.
You could have two circuits hots tied together. I know you might be thinking that if you put two hot wires together they should short out and spark, well thats half true.
A panel has two main hot wires, sometimes called phase A and phase B. Those are your main black and red wires coming into your panel. Phase A and B travel down buss bars to your breakers. So each breaker is either fed with Phase A (black) or Phase B (red). Anytime I take Phase A and touch it with Phase B (with no load in-between like a baseboard heater) you get a short circuit. Sparks and a breaker should trip. But if you touch a Phase A with another Phase A with no load in-between than they WILL NOT short out! NO sparks and NO breaker tripping.
What happens in some junction boxes (such as a light or a plug box or anywhere wires are connected to each other), a person might accidentally tie two circuits of the same phase together. The circuits individually can both still work like they are supposed to, and the ammature person who hooked up the circuits doesn't see sparks, everything works, and they assume they now know how to wire circuits! (You are never supposed to run multiple circuits in the same box without something to warn people and dividers etc.. in the box to separate the circuits, but people still do it.)
So thats a long answer but a way you can test to see if this is going on is-
When you test each circuit individually and turn the circuits back on after testing each one (so only one breaker is off in the panel at a time) and your circuit never went dead,
BUT if you shut off the main breaker for the panel you are testing breakers out of and the circuit is now off, than it's probably two hots on the same phase tied together. Now you must try combinations of two breakers off at the same time to get your circuit off!