Found a redundant live cable during building work

[SUNRAY]

Next to useless, and proves nothing.
Do not rely on that for anything.

Why do you constantly criticise such useful tools?

I carry LED (also functions as a voltstick and live wire locator) and neon scredrivers and use them all the time, I have worked in situations where using both before touching anything (anything includes things like water/gas pipes, handrails and metal stairs etc).
LED screwdriver is my most used device for faultfinding in control panels at mains or 24Vac, they are a godsend in spaces with multiple supplies.

They don't of course replace other test kit but they complement it exceedingly well.

One example I give: I was on a job as 'the boy' with a sparx and while there asked to look at the sockets in a room that didn't work, he proved dead with his big yellow multifunction tester which showed a L-N-E short. N-E short was expected and seeing no voltage present assumed missing and shorted L, confirmed lack of voltage by testing to light switch screws and electric panel heater on SFCU.
By chance the only screwdriver we had in the room was the bigger of these

which lit up as soon as touched on a socket screw or the heater.

The light switch was a plastic PB box so no earth.

The actual fault was a cable damaged while installing slotted shelving fittings directly above the socket which by absolutely bad luck had broken N & E and shorted the load side to L.