This weekend, in the rain, with a full car, I put my camera bag in the trunk of my '05 S60R MT and used the grocery bag holder to keep the camera from bouncing around. When I went into reverse the backup warning began to chirp like a cell phone ringing. Groceries don’t do it. Any idea why it happened (several times and every time I was in reverse)? By the way, being able to disable the system is a very nice feature when it malfunctions.