:Hi, : :> like a headlight, heated rear window, electric window motor, air con fans :> running. :> Its not going to anything like radio memory causing it. :I agree. All of these are visible or audible (fans, lights...) excepted one :: remove the bulb(s) used for the trunk lighting. It may be a cause, and it :was a common fault with some people : they go fishing, leave the boot :opened, and in the evening, when they try to get back home, no power. : :HTH,
Either a fan or headlight remaining on would be easily detectable but a trunk light would not be so evident. On the 307 the rear seats fold down so it would be easy to check whether a trunk lamp was permanently lit. Even if it were a trunk lamp it would take at least a few days to drain the battery, particularly where the vehicle is being driven periodically.
I had a sedan once and a small panelbeating job had to be performed which involved adjustment of a trunk support strut. The repairer used an oxy torch to soften the strut to bend it before repainting the strut. He checked his adjustment by shutting the trunk immediately after the heat treatment and when he opened the lid the lamp was on as expected. But he failed to notice that the retained heat in the strut had melted the nylon plunger on the trunk lamp switch. When he closed the trunk lid again the plunger could no longer open the switch contact and the lamp remained on permanently without anyone being aware of this fact. It took about 8 days for the battery to go flat and I thought it was just the battery getting a bit old. I recharged the battery to get it started but again it was flat within a fortnight. Only then did I start some checking by measuring the standing current drain on the battery. It was over
500mA (6 Watts), which was abnormal, so I did some checking for lighted lamps. When I opened the trunk and felt the bulb immediately it appeared to be excessively hot for only being lit for a few seconds so I checked the switch and saw that the plunger was completely melted away.