The System Notification Helper pops up a window to notify "Software upgrade notifications are available" every few minutes for the duration of every login session, even though no updates are actually available if one goes to Discover and checks.  

This began happening about 9-10 months ago under Kubuntu 16.04 and has persisted across all updates since including upgrade to 16.10.  I've installed/uninstalled various update managers over the period with no effect.  It isn't clear to me which component is responsible for generating the notifications so I am filing this under the Discover updater.