Coming from a rather small clan (MacQuarrie) I would be more likely to assume that someone wearing my clan tartan was a clan member. That being said, I frequently wear my SWK Leatherneck even though I was never in the Marines, I do tell those who ask that I am wearing it to honor my brothers service during the Korean war. If someone is wearing a tartan in an inappropriate fashion that does tend to irritate me, but I am not the tartan police, so I write it off to their ignorance and go on my way, leaving them to be snickered at by those in the know.