The earths magnetic field is very weak. So weak, that holding a compass near another compass will give you a deviation already.
Instead of using a vague iPhone app which may or may not work, why not simply use any old handheld compass and move it about the cockpit and engine bay? You can get compasses as gimmicks in children's toy stores these days for next to nothing and they'll probably work just fine for this purpose.
In fact, with an actual compass it'll probably be the North or South indicator which will point straight to the offending item. Like a magnet drawn to it - and in fact that's exactly what's happening.