My wife's Mazda 3 bought new in March 2007 sheds small black particles down the rear hatchback when washed. Car is garaged at home, not parked in all Scottish weathers, and car is always hand washed at home using car shampoo. The likely culprit is bits of the seal of the window in the rear hatch although no leaks inside have been noticed yet.

Is this something that is known on this model and should I spend money getting it fixed before leaks start? Is there some preventative measure that could be used to stop the rot?