If the mains isn't coming on periodically, which it probably isn't, then the radar explanation if probably the most likely cause. Early warning radar and radar sweeps from nearby airports can cause this. The other possibilities are -
mains-borne networks - these use high frequency fm signals to transfer network data down the mains cables and can cause problems such as this. I've even seen it when the neighbours have a main network, even though I haven't!
plasma / lightning lamps - these can generate HUGE rf fields that can be felt up to 10 feet away ! If you've got one of these, try holding a neon lamp close to it and see the light show!
neighbours' wireless network - these can transmit up to 300m !
near-by cellular phone masts - miles !
near-by radio masts - 10's to 100's of miles depending on the power
There's probably many other possible causes, when looking just remember, for the bulb to glow the r.f. field must be high frequency.