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I have a 2000 BMW523i in which the battery goes flat whenever I leave the key in the ignition. The ignition is in the "Off Position" but the battery goes flat. It has been suggested that a relay switch remains open, but the BMW dealerships cannot solve the problem. The problem does not always occur.
Why ?
I know the simple remedy is to remove the key from the ignition, but I admit to being forgetful in this respect, as security is not a problem, when the car is in a locked garage.
Does anyone have any suggestions

We suspect the wiring system (called a 'bus' system by many) is staying alive when the key is left in the ignition regardless of its position.

Strange as it may sound, it may be a normal condition for this model but BMW should be able to confirm that one way or the other.

If it is not a normal condition, then it really is a job for a repairer/auto electrician who specialise in this brand or are familiar with the on-board wiring system this model has.

At the end of the day it may be cheaper and easier to come up with a way to remind yourself to remove the key completely.

A possible cause of this could be the key itself. We have an 2002 3 series and the battery in the key (for locking, unlocking, alarm etc) charges when the key is in the ignition and I think this features dates back to 98. This helps prevent the battery going flat which results in the key having to be reassociated with the car which is a bit of a drama and needs to be done by BMW. I would have thought that the charging would only takes place when the ignition is on but that may not be the case and 12 or so hours of charging may be enough to drain the car battery enough to make starting hard/impossible, especially if the battery is past it's best. That said, I thought BMW would be aware of this charging feature? Good luck getting it sorted.