thought I'd reply to myself here and state that I've actually run into this
I used to have an online shop with some t-shirts I made up. One t-shirt contained a picture of a primered bumper, with no distinguishing (car-wise) features, and no logo. I got a C&D letter from Ford insisting that I remove it as it infringed on their trademark (the bumper was NOT from a ford car, nor from a ford-owned brand).