...it might just be a forgetful person.
And I really wouldn't have minded if someone had gently stopped me the first 3 times I wore this skirt, "Miss? [or ma'am, which I seem to get more and more - it's disturbing] Your back vent is still sewn shut."
Really. Stop the next person you see with sewn together pleats or vents - it's the genteel (and kind) thing to do.
I'd put it on by the time I noticed, and didn't feel like taking it off, so asked H to snip the threads.
Now I'm perfectly presentable.