I have a Greenlee GT-11 voltage detector, practically new. It's about the size and shape of a magic marker, but instead of a felt tip it has a flat blade with which to probe for live voltage. Stick it into a receptacle or even touch it to the outside of a live cable and it flashes and beeps. At least, it used to.
Last night, I applied it to a cable I was pretty sure was live and it was mute. I then tried it in a receptacle I knew was hot and it was still mute. I tested the batteries. They were good, but I replaced them anyway. Still nothing. I then resigned myself to buying a new one and tossed it into a box of batteries, where it immediately began beeping. I took it out and started testing it again and it beeped accurately, until I turned it off and back on, then nothing again.
So, the problem appears to be in the switch, though the thing is so simple I don't know how it could be failing intermittently.
Bottom line: Once the thing is on and known to be working, it is reliable until turned off. Once turned off, it has to be tested on a known live circuit when it's turned on again. (Which is recommended practice anyway.)
My concern now is: Once it's on and working, can I depend on it to work for as long as it's on? Could it just stop working without warning?
Unless someone has some simple solution, I guess I'm off to buy a new one.