In the Notification area of the desktop we get a notification about an available update but opening Discover or even using apt or apt-get from the command line shows that the system is up to date.
Upon opening KDE's Discover we see "No updates".
P.S. I just helped someone with this issue so I thought I'd post this to help anyone having the same issue as there seems to be no question here about it.
This is a known bug with Plasma that is reported to be fixed in a newer version of Plasma 5.16.0 which is not available by default on versions of Kubuntu lower than 19.10.
Kubuntu 19.04 is currently running 5.15.4 and 18.04 on 5.12, so you may encounter this annoying bug.
The bug is related to flatpaks. If you have flatpaks enabled in KDE's Discover, at least some packages are not update-able via Discover.
You can easily workaround the issue and fix in the terminal by running
This will update the outdated flatpak that Discover was unable to update and should make your persistent notification about a "phantom" update go away.
In this case, the flatpak affected was an Nvidia driver but the bug reports mention others as well.
Use the info below only if you are familar with Linux, it is not a noob friendly section and only included to add some info from the comments
As mentioned in the comments below you can use Kubuntu Backports PPA to get a recent version of Plasma that will remove this issue, but it is very much a use at your own risk solution I could pull in less stable versions of Plasma packages that are still being tested. Only use this if you are familiar with how to purge PPAS and are willing to take the risk. (also this ppa should be purged before upgrading to the newer versions of Kubuntu to avoid problems)
I used this backport with no issues but your experience may very. I am not offering this as a solution only as information for the braver users. This warning is a copy and paste from the ppa description: