I've had a textbook set off the passenger seatbelt gong. Weighs about 5 pounds. A book is a little over the top for sensitivity but a cellphone is way over.
2013 F30 335i | Sportline | Imperial Blue Metallic | Black Dakota | 6-speed | HK | Cold | Premium | 19" Staggered Sports | M-Performance Exhaust
2009 E90 335i
2007 E90 328xi