Odd problem. I want to replace a busted porch lite on the front of my house. Simple enough, right? Problem is, I have not been able to shut the power down on that line. Two times I ran through every circuit breaker on the panel in the garage, and that light never shut off. To my knowledge there is not another panel hidden away anywhere else. How can this be? And while I realize I can shut the main line, (which I haven't done yet because other people were in the house and needed juice), what could be the cause of this?
PS: can someone recommend a simple testing tool to see if a line I am working on is live or not? Thank you.

The box stores sell a simple tester that does not require to touch live liness, cost about $10.

If it is on a breaker, then the breaker may be defective. Fixing this is dangerous, must open the panel and shut off then test each breaker to find the one that stays on with the handle off.

Certainly must shut off the main to test to be sure the main shuts every off.

Anything beyond this anything is possible. I really think you may need a pro to check this, unless you are comfortable working around live parts.

Remember, this stuff maims and kills. An electrical worker just got electrocuted in Vermont,
Another brother down.

It is probably powered by at least 2 Breakers(usually2) feeding the same circuit.
Turn off EVERY breaker, It should go out, if it does, then turn breakers on one at a time.
When it goes on, identify that breaker, turn that breaker back off, continue to turn on breakers until it comes on again, identify and turn back off.
To be safe you have two options.
1)with power off both? Remove wire from 1 breaker and cap off.
2)isolate both circuits(some where, 2 hots from 2 circuits are connected.)
If nothing is done there is the possibility of not tripping breaker at 40 Amps.
(2 20 amp breakes in parallel)Potentially Very dangeous.
Good Luck

If you have a voltmeter to test circuits or you can use what they call a ticker go to any electrical supply store they should have it the ticker would be the best thing to use because you can just touch a wire with it to see if wire is live or not. If and when you get either tester you must remove the cover from the panel. As you know at this point your life is on the line if you are not careful. Just for kicks check to see if any of the circuit go to the main lugs. If they do remove and put on different circuit be sure you are careful. Now test each circuit with tester with the breakers on and with the breakers off one at a time to see if circuit goes off. If you get no power to all breakers your light should be off. If you get power to one of the breaker when off that’s your problem the breaker. If you are still getting voltage to that light call the PROS.

Another way to do complete test at panel
Turn all breakers off.
Turn on one breaker, Check for voltage at all 120 volt breakers in the off position, should be no power on any breaker being "Backfed",
Turn on next single pole breaker, verify no voltage at all off breakers, Repeat all 120 volt breakers(no need to check previous breakers) If you do, Isolate or correct as above, good luck.