I would guess it's a quirk of your browser (probably newest Chrome?)
I think the issue is the forum avatars (the little profile pics that appear on posts and on your profile when signed in). They are not HTTPS links, but are HTTP, and pretty much all browsers warn on that these days (mixed content - HTTP content on an HTTPS page). That seems to be what Firefox here is warning on.
My Firefox 31 and Firefox 52 browsers note it with a ! symbol next to the address bar on BOTH signed out and signed in pages. There aren't really any difference between the signed in or out pages (other than the slightly different layout, the addition of your info at the top, and some extra cookies dropped).
Another reason I suspect you are using Chrome - there were indications during the beginnings of the mixed content hullabaloo that Google was differentiating between authenticated pages and non-authenticated, and may be warning differently on them as a result.
[rant]Chrome in particular is going way to far with their quest to fix the internet's security problems RIGHT NOW. Security is good, but there is 20+ years of lax security practice, existing code, and inertia to consider - which Google does not ("we have largest browser share, we make changes, users upset, users complain and mount viral media whinefests, force internet to comply to make warnings go away, cause omigod hackerz") .[/rant]
