I just saw this info on the web and thought it might be of interest.

Secondary battery continually going flat on 2008 Bentley Continental GT. Dealer cannot find the fault. I suggested: An electrical circuit remains live. The first suspect is always the interior light 'see you out' dimmer switch. These often remain live after the light has dimmed and gone out because the circuit remains live. To check for that, switch off the interior lighting completely. If it solves the problem you then know the source. The second suspects are the lights in the trunk and the glovebox. Best way to check if the light goes off when the lid is shut is to set a smartphone to video record then shut it inside the trunk and then the glovebox. If the video shows a light on, you then know that those switches are not working. The third suspect is the Bluetooth system. If a car is not locked properly the Bluetooth will continuously search for its paired phone using power. Or locking the car might not switch off the Bluetooth. The fourth suspect is the alarm/immobiliser system and particularly if some form of tracking device has been installed that takes a continual drain from the battery. The fifth is not driving the car far enough to recharge the auxiliary battery. If the car is used only for short runs then this could be the reason.

 

Using a smartphone video. Creative idea!


It seems to me if you have your boot open for quite a while, like providing for the electrical cord to the battery charger, the light in there will turn off. If I'm correct then you should be able to test if your boot light turns off by just leaving the lid open for 5 minutes or so.

 

I like the smart phone idea as well