On 10/12/2017 22:55, D.M. Procida wrote:
I noticed that the wire into a ceiling lamp lit up an electrical
screwdriver when I checked its terminal, and it did the same at the
switch end - even when I completely disconnected it from both ends.

I'm pretty sure that this was a single uninterrupted wire from switch to
lamp, with nothing else connected to it, but I can't be 100% certain.

I assume that there was a very low induced current in the wire from some
other nearby wire; whatever it was, I couldn't actually feel anything
myself when I touched it.

Is this normal behaviour for an electrical screwdriver, to be so
sensitive?

Yup. I've got one of those table lamps which turn on when you touch the
metalwork, and that lights a neon screwdriver dimly, showing that they
put mains (or near mains) voltage through you. Real electricians dislike
them as they respond to leakage currents.

--
Max Demian