But have you ever tried using one on drywall with metal studs? It goes crazy and beeps at everything, even ductwork or pipes. Those old plaster walls are basically the only place they work right. For marking, just use a tiny bit of painter's tape and make a pencil dot on that.
Tell me about it. I tried scanning a wall in my kitchen last week and it lit up like a Christmas tree over every single bit of conduit. Ended up with so many false positives I just gave up and used the old knock-and-listen method. Those things are basically useless on any modern construction.