energy saving bulb glowing intermittently?

Following on from a question by Al.
We get exactly the same symptom :-
the bulb glows very faintly every 20 seconds or so.

However the light is NOT switched on.
When the light is switched on the bulb works normally.

I think this is the same as Al - he took out the fuse to stop it - but the response seems to assume that the light was switched on and the bulb not working correctly.

Any ideas what would cause the bulb to glow when the light is switched off ?