Partlet worn open is just a thing women did because it looked pretty. Since there's a crapload of pictures of married and unmarried women all wearing them open (the Fete at Bermondsey painting has a woman with an open partlet/shift breastfeeding, and that's *not* something an unmarried woman would do in public - they'd be hiding the baby and pretending they didn't have one), I think it's safe to say it's a fashion, not a statement of marital status.

And you can tell them Isobel said so. Nyah.