I first saw this bug on the macOS 15 Sequoia betas, which I ignored as a beta issue, but I've started to see the bug on Sonoma too after updating to macOS 14.6, and the bug continues on macOS 14.6.1. I've seen the bug on both my MacBook Pro and my Mac mini.
Out of nowhere, an "Updates Available" notification appears, despite the fact that I already installed the macOS 14.6.1 update yesterday.
When I click the notification, it opens the Software Update pane in System Settings, which checks for updates and finds… nothing. "Your Mac is update to date."
Note that I use Little Snitch to prompt for connections from the softwareupdated process, but there was no prompt before the "Updates Available" notification. After I clicked the notification, there was a prompt to connect to swscan.apple.com when System Settings opened.
The value of defaults read com.apple.SoftwareUpdate UserNotificationDate seems to be the same as the time that the false notification appeared. Perhaps my trick to stop Upgrade to macOS Sonoma notifications, i.e., set the date to the far future, would work in this case too?
Apple's software quality assurance is at an all-time low. We can't even avoid beta bugs by sticking to the so-called "stable" releases, because now Apple backports the beta bugs to those (un)stable releases. It Just Breaks™.