When I was trying to use a http outgoing link to the manufacturer it was causing SSL warning unsecured items on page. That was located in the top left sidebox. I remove the link from it and all was fine. The pic I am using for the manufacturer also was causing SSL warning, but I simply added https to the picture link and it was ok on non ssl and ssl pages.
But yes, outgoing http links will cause SSL warning, at least it does with my SSL and my site.
I also have the entire www.sldingglassdoorparts.com domain SSL not using SSL subdirectory.
Zen Cart adds the https to pictures and links for product pics and internal links when I go to https pages, but if some link has no ssl adding the https to that link wont do any good.
I am told by HostGator that the reason I get the warning on regular http pages for http links is because the entire site is SSL.
I also have phpbb3 installed i a directory on the site and have not tried getting it to work yet. It may not do well in the SSL enviroment, I'm not real sure yet.