R129 SL500 battery woes

[Roast Beef Racing]

The battery on my R129 SL500 keeps going flat.

The car is kept in storage over the winter on a C-TEK. I noticed the C-TEK had a red light signalling there was an issue with the battery and it wasn't charging. The battery can't be more than 4 years old but I replaced it as it was cooked.

Today, 2 weeks after changing the battery for a new one, it is flat again! It has been on charge but this time with a different C-TEK so it's not that. The car hasn't moved anywhere since I replaced the battery but I did start it to reset the windows when I replaced the battery.

Any ideas where to look for the issue?

[Roast Beef Racing]

So...it's not the boot light, the vanity mirrors, ariel or immobiliser (doesn't have one). I used a jump pack to start the car and it seems to be holding charge. Very odd. Will get an auto electrician to take a look at it when I have time.

Thanks for all the suggestions. I remain bemused.