The panel (configured as auto-hide) sometimes pops up for no reason that I can see – it's not due to a notification, for instance. When this happens, the panel stays popped up until I move the mouse over it. This has been happening once or twice a day since upgrading from 4.9.5 to 4.10.0; it didn't happen before.
I've realised the panel pops up 10 minutes after a file transfer, when the notification about the transfer is removed. If this is deliberate, it seems useless to me.
It also pops up 10 minutes after adding a new RSS feed to Akregator. Maybe all notifications, but I can't reproduce it with "kdialog --passivepopup 'This is a notification' 5".
Plasma 4 has been unsupported for several years. Please file a new bug if it still happens in a recent version of Plasma 5.