Strange electric situation - advice?

If the OP shut every breaker in the panel (at the same time), and there was still power, there is a big problem. Someone must have wired directly to the main, or else there is a hidden panel somewhere. Anyone that knows wiring would be able to trace that circuit back to the panel. I always try to support people to DIY, but if the OP cant trace the wires back to the panel, I suggest he call an electrician or a friend who knows wiring. If that circuit is not on a breaker, it's a fire hazzard. You dont connect a #12 or #14 wire to a 100A (or more) main. An overload will set that wire ablaze in seconds. If this was my house, I'd find the cause no matter what.

Maybe I am not understanding this, so here's the way to test. I am assuming he shut off ALL the breakers at the same time. Then one by one he needs to turn them on (only one at a time), and shut off the last one that was turned on. In other words, only ONE breaker turned on a a time. If TWO breakers made the same lights turn on, there there is a crossed circuit (which is bad). If shutting off ALL the small breakers (at once) does not turn off these lights at all, there is an even bigger problem, as in unprotected circuit. Of course he has to connect the light back to the switch or connect a meter or test lamp to it. Working on a 10 year house should not be that bad. It's not like some of those really old houses with the knob abd tube nightmares.

A couple alligator clips on a bulb socket with a 25W bulb makes a great tester. Costs under $5 to make one.

I'm a retired electrician. I've dealt with things like this, and it needs to be fixed or can be very dangerous.

Granpa

Nothing. The reason the question was asked, though, is that if you had been using a digital meter, you might have been reading induced voltages at very low current ("phantom" voltages), and misled into thinking that the circuit was on when it really wasn't.

Yes, that's possible, maybe even likely.

Physically trace the wiring from the lights back to the breaker panel.

Oh how many times have I said that and found out otherwise.

Have you tried disconnecting all the work you have done and then see what happens?

They could, and yes.

Think about it. 120V loads are ALREADY connected to the main feed (a breaker is not a transformer). Any other 120V load could be connected the same way, although this is not desirable.

Since it's already wired incorrectly, you can't be sure it's actually hot that's being switched.

Ok, so now the dramatic (or not so dramatic) conclusion. I did buy a circuit tracer, and hooked it up. There were no outlets on this circuit so I connected the adapter and tied it directly to the screw terminals on the switch that was left in the box. This worked okay, but the tracer couldn't pinpont a specific breaker, since it would indicate 4 consecutive breakers as all being the correct one. I guess I was a little confused at first too since this is a 3-way switch and maybe wasn't sure which lead would be "hot" when the switch was on or off (and the other switch too). I can follow instructions to install a 3-way switch, but troubleshooting an existing one and not knowing specifically where power comes into the branch is a little beyond me. So I simply took the switch out of the equation and disconnected all four cables coming into the box from each other. One went out to my lights that I'm trying to disconnect and has no power, even with the breakers on (the lights are all downstream of the switch). One goes out to the 3-way switch, and has no power of its own. The other two, as suspected, each have power from their own circuits. One ties back to a lighting circuit - main panel breaker 1A, and the other ties to a "Smoke Detector" circuit - main panel breaker 2B. I capped off one of the power feeds and reconnected the other to my 3-way switch, taking the second switch out of the box. Now, all lights that are supposed to work do, smoke detectors all function appropriately, and the lights that I want to remove have no power. Thanks to everyone who contributed to help me solve this error and potentially dangerous situation.

Did you separate the two white supply conductors from each other so that each one only serves the loads supplied by the black wire from the same cable. If not you will want to go back and do so. This will prevent either one from being overloaded by the current from two breakers if the white wire from one of the circuits goes open up stream at some later time.

-- Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

Yes, I capped off the hot, neutral, and ground wires from the circuit that was servicing the old lights. I re-checked all breakers and now each one serves only one circuit. I'm confident that I fixed this error on the part of the original electrician (or previous homeowner), but it does make me wonder how many other mysterious and potentially dangerous conditions exist within everyone's home.