I haven't read the whole thread, sorry, but, yes, OP, this happens to me at least once a week. I work in a school so there's a whole lot of plastic chairs.
About a week ago, I was at a conference with colleagues. I sat with two male colleagues who have all joined the school recently. We were sat down in a warm room, on plastic chairs, for about an hour and a half for the welcome and keynote speech. When we got up to leave, I had left the triangle too and as the chairs were pale blue it was fairly noticeable, but I thought it was better to just walk off than draw attention to it by trying to wipe it away (and I had nothing to hand to do that with, just my laptop).
Unfortunately, as we were standing up waiting to walk out of our row, one of my new colleagues dropped a bottle of water he was holding and, of course, it fell underneath the chair I'd recently vacated. The two of them both then bent down more or less with their faces on top of / level with the diamond of shame and spent what felt like hours scrabbling around for this blasted bottle of water. I was mortified. Neither of them appeared to notice the mark, though.
Maybe they thought I'd wet myself. Or maybe they genuinely didn't see it? Maybe they thought the chair had an existing mark on it? Or, my final thought, which is rather far fetched, is that because they're both Biology teachers, they were cool with it?