Connect an amp meter in series with your battery, i.e. remove one terminal then connect one lead to the terminal and the other to the battery, the meter needs to be set at 10 amps, make sure everything is turned off first. If there is no or little reading on the 10a range switch to 1 amp, the reading shouldn't exceed around 100ma, if it does pull out one fuse at a time until the reading drops then check what that fuse feeds.
I had this prob soon after purchase of c5-2007 vintage. Update at dealer sorted it, it was the audio system. If your car is this age do some research, call dealer and see if its missed an update. Probably unlikely this mis ypur prob but worth a try.
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