I've noticed in the last two houses I've lived in that CFL bulbs will periodically flash even when not turned on. It's about one flash a minute, and it's not very bright. You can only notice it when it's dark. I originally thought it might be power in the CFL circuitry leaking out after being turned on, but the flashing continues even if the lights haven't been powered up for days.
The wiring in both houses was old to start with, but then redone with new wiring, RCDs and so on. There is no voltage between the contacts in the socket when I check it with a multimeter, so how are the bulbs getting power to maintain a constant flash?