I think there could be issue.
Sometimes I have noticed the icon to be green, but still there can be updates.
I just assumed that this is because new updates become available between me starting the computer and when I clicked the icon. Because I don't really know how the software checks the updates.
Asus P7P55D, i5 750 2.6Ghz, 4GB DDR3, radeon 6540 (Gallium 0.4), 80GB Intel SSD, Dell 1920x1200, Dual boot, Linux Mint 14 Cinnamon, Windows 7