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Battery discharging

My 2005 Cad.STS has on two different days discharged the battery completly overnight. To my knowledge, no lights were left on overnight to run the battery down. The first time, I recharged the battery and had it check and it was good. The second time, I recharged the battery and took it to my dealer and they had it for two days and cound not find anything causing the discharge. Has anybody had this problem and found the item discharging the battery? Thanks for any help.

Are you sure the charging system is OK?

Check the obvious. Trunk light, brake light, glove box light, underhood light. If it kills the battery in less than 24 hours it’s probably pretty blatant.

It’s possible the battery is shot.

Yes, and the easiest way to test that is to remove the negative cable from the battery when you park it for the night, if the battery is still discharging overnight, it can be nothing else but the battery.

Battery and charging system are a-ok.

The auto-matic shut off switch is working. The dealer says there is a short somewhere, but they do not know where. They request I have car hauled to dealership when this happens again before recharging the battery. This has not happened yet, but you never know.