My guess is you have been looking at the bottom end of the market when it comes to stud/voltage detectors. It is the same old story.. 'You get what you pay for'.. this is the one I use and it will get replaced when I break the case or the screen, but it may well have saved my life a few times.. it does work !
http://www.axminster.co.uk/bosch-gms-120-wire-pipe-and-stud-detector-952294
In an ideal world all wires will travel up and down a wall and across only when supposed to, but the last thing you should do is trust someone else to put wires where they are' supposed to go'.. I have found wires bady positioned many times and if you think that driving a nail into a wall to locate them is a good idea then sort your will out before you attempt it. I know you are hoping to find a batten/stud, but the nail doesn't know that.
I often fitted handrails on behalf of Social Services.. not long ago I was asked to fit one urgently as it had been partially fitted and needed completing the same day.
When I got there I found a tradesman's tools scattered around the stairwell. On asking the resident what had happened, she said another man had started fitting the rail earlier in the day, all of a sudden the lights and telly went off and there was a load of banging and crashing as the fitter fell down the stairs. When she got to him he was unconscious, so she phoned an ambulance.
He did recover and was later released form hospital.
When I started to survey and see how far the job had progressed I noticed a screw stuck out of a handrail bracket which had a black end on it. I got my voltage detector and scanned the area .. the black screw and the wall around it showed electricity present and was live. A neighbour had come in and flicked the power back on after the accident, as the elderley lady had no heating ... I turned all the power off and I set about removing the screw. After inspecting under the stairs I found the heavy 10 amp power cable that supplies the electric shower tucked up and hidden between a gap in the stairwell and the wall where the handrail was fitted (badly routed).
It seems that the fitter was gripping the handrail bracket with one hand while using a screwdriver with the other when he penetrated the cable ... I didn't find a voltage detector lying around amonst the fitters tools when I gathered them up. But there again .. he wasn't expecting to find a cables where it shouldn't be !