Why do I keep seeing a "Not Secure" warning when visiting the ISF?

A nasty looking red warning triangle with exclamation mark keeps appearing next to my Refresh button alongside the address bar at the top of the page, only when I visit the ISF. It isn't on every page here, and can sometimes go when I click refresh, but it's been bugging me all day. I don't think I have seen it on other sites.

What's afoot?