using a B&D Marksman AC Sensor

I am trying to figure out how this handheld unit works for detecting power. I don't have a manual. I am disconnecting and removing electric baseboard heaters. I just want to figure out which fuse controls this power circuit. Even with all the breakers thrown my handheld AC Sensor gizmo seems to beep furiously as if there were power. I know there is not because when I turn the heater switches ON they do not heat up.
Can anyone tell me how to use this thing to determine if the power lines are live? (working intuitively - admitedly not a good idea with electricity - I hold the side button down and pass the unit over the leads. But it still beeps.)
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If you're depending on that piece of junk to verify voltage, then you are - quite literally - taking your life in your hands. Either buy a volt/ohm meter and learn how to use it properly, or get someone who knows how to use a volt/ohm meter to come check it for you. It's not worth risking your life over ... you could have a separate panel box that controls the baseboard heaters, or some jackleg (or homeowner) could have done some "creative" wiring ... when dealing wtih electricity, it's always best to err on the side of caution - and caution in this instance would tell you that there's still power on those heaters!!

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thanks. good advice. I hired an electrician to verify and disconnect everything.
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