Half way through our refurb we had to change electricians as the original one ceased coming! He was moonlighting it turned out and his employer found out. Good thing probably as we have found numerous dangerous things he'd done ( such as using the earth /cpc conductor as the switched live on our shower light/extractor).
Latest thing we found was two 'wired' smoke detectors not connected to mains or the other detectors. Carpets newly down, ceilings freshly painted, no access.(He'd assured us everything was tested and ready to seal up) New electrician was flumoxed how to proceed. I got out my telecoms wire sniffer comprising a transmitter/oscillator intended to connect across a pair, clipped it to live and neutral of the isolated detectors and was able to trace the wire with the sniffer receiver across the floor, up a hollow partition wall and into the loft under a mass of insulation that would have stopped anyone finding it by normal means. He'd left it coiled up forgetting to connect into the adjacent detector. Mistake by prat overcome by technology
AWEM