Last night in the pitch darkness part of a tube on an 'energy saving' light bulb began to flash faintly. WHY?

Question

The bulb had not been switched on since May.

The switch was fully off.

Faint flashing was at 20 second intervals and did not stop.

The flashing has not been apparent before tonight.

Answer 1

To me it seems like there might be a problem with the switching device. You did not state where the bulb is located so it makes answering your question a little more difficult.

If in a floor or table lamp the cause might be related to the socket or possible power surges.

Recessed lighting -Possible fixture issue or switching device. Are they on a dimming switch?

Flourescents don't magically come on without some sort of power. Further investigation might be a good thing to think about.

Answer 2

CFL?  Many fluorescent tubes do this and they will keep working just fine for a long time after.  If it bothers you, change the bulb.