School Of Hard Knocks Dept.
When using a non-contact voltage detector (looks like a carpenter's pencil)
Never ever EVER use the device without giving it a verification test first.
Even if it means walking fifty feet to find a proven working circuit, verification can be life-saving. Even the most expensive non contact testers have AAA battery power integrity issues.
I wrapped a thick rubber band around mine. Not only does the rubber band serve to make the detector harder to slip out of a pocket, it serves as a reminder to verify proper operation at the start of each project.