A detector is good to see for example where there are wires or joists in a wall. You get great flexibility since it runs on a battery and lacks a cord that can restrict your movements.
From Amazon
Bosch Professional 12V System Wall Scanner D-tect 120 (12V Battery, max. Detection Depth Plastic Pipes/Wooden Studs/Live Cable/Magnetic Metal/Non-Magnetic Metal: 60/38/60/120/120 mm, in L-Boxx)
Conrad Electronic
Bosch Professional Detector D-tect 120 0601081301 Locating depth (max.) 120 mm Suitable for Wood, Ferrous metal, Non-ferrous metal, Live wires