I had this issue as well for a long time on my 3.0. Sounds like it was a little different than your issue, but mine was after I would lock the car and walk away it would randomly go off a couple dozen times, sometimes it was when I would unlock it too (and who knows when else that I didn't hear it!). The dealer hooked it up to the GT1 and found it was going off from all sorts of sources, ended up that replacing the ignition starter switch was what finally stopped it from wreaking havoc on my neighborhood!

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Vehicles:

1. 2005 4.8is X5 Sapphire Black Metallic/Black Leather, fully loaded (Nav, Comfort Seats, Rear Climate, Sliding Load Floor, Heated Front and Rear Seats, Aux In)
2. 2004 3.0i X5 Jet Black/Black Leather (Lemoned)
3. 1999 Saab 9-5 Turbo (Trade In/Retired)


My Custom X Page (Pics of my 4.8is)