Fine yesterday, tried opening the boot this morning, nothing. Central locking had unlocked. Start button flashing yellowish, no dash or power to starter. Fluke says battery at 4.5 v which means it is dead and due for burial. Started on jumpstart but not happy, fuel gauge at zero and dials erratic. Off to dealer to be fixed.
Well its back and everything seems to work. Apparently it has had a software upgrade as the electronics where not totally shutting down once the ignition was off and the current drain was enough to totally flatten the battery over a period of days with only minimal milage, still doesn't explain the 4.5 volts though. We shall see (keeping booster battery in the boot for a few weeks).
If it's that low a voltage after putting into the car, but holding charge out of the car, I would leave it disconnected until they can fine what is shorting out. That's a huge amount of voltage drop, and the next thing will be a fire.
However, if it's just draining down to 4.5 volts (which it sounds like it may be), that's also a very heavy drain and the battery won't last too long repeatedly draining down to that. A clamp on ammeter would definitely read something significant.
Personally, I would suspect the battery (best case scenario, and also the most likely).
Edit: also, I would concur with it being a 'boat anchor' - who uses unsealed batteries in cars?! Sounds crazy to me (but probably very cheap).