Is it rude to point out a slipping hijab?

I am not Muslim, far from it, but I live in an area with a surprising number of women and girls who wear hijab head coverings. I will often run into them at the grocery, or in the library where they are child wrangling, etc, and notice that their hair is slipping out or their covering has slipped far back. Is is rude for me, as a non-Muslim woman who is likely in shorts and a t-shirt to point this out, or is it likely the woman in question will appreciate it? Is this the cultural equivalent of an exposed bar-strap, or a wardrobe malfunction? Genuinely curious and like to be neighborly, at least in Real Life.