Using Xubuntu 13.04 x86-64, the last two days when there have been updates available and I've selected "show updates" from the notification icon, the list has been empty, but it reports (today) that 195k will be downloaded and the install now button is lit up. When I run apt-get upgrade it shows that there are in fact updates to install, today it's
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be upgraded:
bamfdaemon libbamf3-1 libdvdnav4 lsb-base lsb-release
5 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 195 kB of archives.
After this operation, 16.4 kB of additional disk space will be used.
Do you want to continue [Y/n]
I did it from the command line yesterday and everything worked ok so it seems to just be a problem with the gui.Here's a screenshot of the update window:missingupdates.jpg