I have this same scenario on 2 separate machines The plasma desktop frequently shows notifications like this:
An update is available for your system. There are 3 updated packages.
However, when I do any of the following 3 items, no updates are found
- at the terminal:
sudo apt-get update
- run Software Updater
- run Muon Update Manager
I am running 14.04 LTS, with KDE.
I have tried
sudo rm -rvf /var/lib/apt/lists/*
which is mentioned in several of the posts which come up from this question, and have read many trying to find out how this could happen.
Somewhere the different tools, notification manager, and updaters must be using different lists against which they check for available updates.