I have had a couple of those alarm-go-off-for-no-reason moments... I will look for spiders now. (mind you, in my case the windows are up all the time.)

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Currently driving: 2011 E70 x35i , 2011 E90 335d
Previous BMWs: 2007 E90 328i , 2005 E53 3.0i, 2011 F10 535i (6spd manual)