Strange indeed, as /backports was never enabled by me. In the package manager, when I select "Backports" and "Installed" instead of "All" and "All", no packages are listed. The installed sync-engine has "Medium: None (installed)" in the package manager. Hmm, when I look through the list of installed packages a lot of them - if not most of them - have "Medium: None (installed)". That's not how it supposted to be, right?
When I try to update the package sources from the package manager the backports are listed as well, does this indicate they are active? The testing repositories are not shown in the list.