Car's use power sitting there with nothing on.
Stuff like your clock, or the wee red light that flashes for your immobiliser, will always drain your battery and at this time of the year, most people jump into their car, turn the stereo on, turn the blower on full, turn the heated rear window on, turn the lights on and expect the battery to be charged for the next time....no.
Get out into the car during the day, take it for a 40 minute drive, then go to Kwickfit or the likes and ask them to do a full battery check on it. It shouldn't cost you anything and they will soon be able to tell you if it's knacked.
Batteries last 4-5 years on average, so i'd think it's just not getting used enough to charge it up properly.