Its called inductive coupling. The wires probably run some distance in
proximity to other wires which are live. Although there is no physical
connection between them the alternating magnetic field produced by the
current flowing in the live circuit will induce a voltage in the
disconnected one. (same principle by which a transformer works)


--
Cheers,

John.



Sense at last!

Yes, you need to be careful with neon screwdrivers. A false negative may
mean a broken screwdriver but if you check that it is working every time you
use it the chance of getting zapped is very low (and if you short every
potential live to earth before touching it it is even lower). You can often
recognise a false positive caused by induction because they don't light
quite so brightly. I prefer my cheapo "electronic" screwdrive which buzzes
on a live because it will do this through insulation, but I still use neons.
And I always check that it's working before trusting it.