Yesterday, I upgraded some plug-ins, and one of them contained an error. As a result, yesterday member contact details were visible on the members page.
I have temporarily deleted everything on that page, and will restore it to how it was later, now that they have corrected the error. I have now installed the correct update, and restored the page.
In future I will immediately check all visible pages after an update to prevent this happening again. Apologies to all for not being quicker off the mark, and thanks to Rich for the tip-off.