Windows says 1 notification for me ... but there are none

Windows says I have one notification:

But when I open the action center I have none:


When I reboot this disappears.

What's up with that, Holmes?

Yes, it reappears. Time after time.
I don't remember what circumstances were in play when the false notification appeared.

Has anyone seen this behavior?

Not seen this behaviour; but try and set "play a sound when a notification arrives" to may be pinpoint the time as soon as it happens:

in case you need the link on how to do it:
Turn On or Off Showing Notifications in Action Center in Windows 10 | Windows 10 Tutorials

Then for a startt check event viewer to see if anything had happened at that time, or use the list of recently executed programs for a hint if any.

ExecutedProgramsList - Shows programs previously executed on your system

I tried to follow the instructions at that link ... but the tutorial must be way out of date. My Notifications & actions screen looks entirely different.

Anyway ...
Show notifications on the lock screen is ON.
Show me Windows welcome experience blah blah blah is OFF.
Get tips, tricks blah blah blah is OFF.
Get notifications from apps and other senders is ON.
There are no other settings.

And at the bottom of the screen it says: No senders have provided notifications yet.
What does that mean?

Yes, didn't realize the link was somewhat old, but now if you click on the the individual apps themselves in Settings > Notifications & actions, it gives you other options including setting a sound.

I don't see any individual apps:

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But if I click on Add or remove quick actions I get this:

I do it this way:

Click
Start Menu > Settings > System > Notifications and Actions > show notiications from these apps

Then, if you click on any of the individual app icons in the list there are further options including "play sound".