An observation - if you add mirrors in /opt/debextra, and those mirrors have their own version of a package that is available in wheezy main, and the package was originally imported from wheezy, then that package will show up as an available update. The extra mirrors take precedence over the main one as that is the desired behavior both in importing and update checking.
So to be accurate during an update check you need to keep the same mirror files that you import the sce with. But if you don't, or add mirrors that contain different versions of packages within the extension before an update check, then those packages will be listed as having available updates though the package in the repo you imported it from may have not been updated.
Example, ssh originally imported from wheezy and then the backport mirror was added before the check:
tc@box:~$ importupdatecheck ssh
#################### 100.0% 0.0 kBps DONE
verifying download...checksum matches OK
used 8798208 local, fetched 0
Using the wheezy-backports main repo of mirror http://deb-multimedia.org
Using the wheezy-backports main repo of mirror http://ftp.us.debian.org/debian
Using the wheezy main repo of mirror http://www.deb-multimedia.org
Using the ./ ./ repo of mirror http://www.pvv.ntnu.no/~knuta/xmms/squeeze
Using the wheezy main repo of mirror: http://ftp.us.debian.org/debian
Checking for updated deb packages, please be patient..
Checking for updated dCore startup scripts, please be patient..
openssh-server http://ftp.us.debian.org/debian wheezy-backports main
openssh-client http://ftp.us.debian.org/debian wheezy-backports main
ssh http://ftp.us.debian.org/debian wheezy-backports main
The above have package updates available with mirrors listed to the right, import ssh again to get them.
No dCore custom startup scripts inside ssh.sce have updates available.
tc@box:~$