The now unobtainable Whites TRX is one of the only true pinpointers - it only detects in the forward direction from its nose.
Most other pinpointers also detect strongly to the side of their head - sometimes upto 50mm up the side . So if you are not aware of this you can still go off track for a while - you poke it down the hole and it goes beep at the bottom & you think ok I need to go deeper and keep digging down instead of across.
They still work fine but it pays to play with some targets and see the field shape and effective distance of your pinpointer so you can visualise what it is telling you.