3 in 1 detector from KWB of West Germany.  Unlike some on the market, this actually works and gives consistent readings! This is the 2015 next generation model from KWB- the previous model won German DIY Product of the Year when it came out, and came top in the Heimwerker Praxis test (similar to the UK "Which" magazine, against a lot of much more expensive tools.
With the new one they have gone from 3 volt to 9 volt battery power, to eliminate the occasional problem with the other one where, if the voltage from the 2 little AAA batteries dropped off in the slightest, the unit went into safe mode and continuously gave a "current detected" reason as a fail-safe measure.

With this unit you can locate studs in plasterboard walls, hidden screws, water pipes, electrical wires and anything else which is different from the main material of the wall or surface being tested.  The unit also gives a separate indication when it is near a live electrical wire.

To use, place the unit on the surface to be tested and switch on.  Wait for the green light and audio signal (time will vary according to the density and thickness of the material- the unit maps the area round where you place it and resets itself to that material every time you use it- this is why it is so precise).  Then WITHOUT LIFTING THE UNIT OFF THE SURFACE move it along the surface and when it finds something under the surface it will beep, the amber light will illuminate when you are at the edge of the object, then the red light will come on when you are right over it.  By finding the point at which the amber light comes one, you can exactly mark the edges of studs etc. so you know exactly where you can drive screws in.  If you get near a live electrical wire, the "AC" light illuminates.  You can then pinpoint the exact position of the cable using the two lights.

If you buy this and have used it for a while, if it starts indicating that it is finding a voltage all the time when there is no electric cable near, the unit is not faulty.  This is a fail-safe feature- if the battery is not delivering enough power it defaults to a live electric alert as a safety measure to stop you potentially using it and getting false readings.  So long as you use decent batteries, they last for absolutely ages.  The instructions are folded inside the display insert in the packaging- this often fooled people with the old unit who discarded the packaging then contacted us to complain that we hadn't sent any instructions!