I've not encountered this issue for a few versions, but it just happened again. I had LS open in IE8 and using a HTTPS. The session to my LS in my browser was probably a day old, and the IIS session I'm pretty sure was "shutdown" (app pool) as LS was slow to respond when I did the following as my first access (pretty sure at this point, it's as it should be - and I did not have to reauthenticate). I entered a user's username in the search bar. As the LS apps spun back up, when it returned a couple answers automatically (dropped down from the search input field), that is when I got the mixed content warning. After this point, I've not received the warning again. I'll try to reproduce again by letting the session "time out".