I use kwrited a lot. I have setup plasma with a vertical bar, 4 desktops arranged vertically, that when switched, generate a cube animation. Whenever the cube animation starts, a popup originally made by kwrited appears, and then disappears when the animation ends. I tried killing and restarting plasmashell but the thing doesn't go away.
It turns out it was an issue with kwin actually. Restarting kwin fixed it. Still, the notification was't even the latest one that had appeared from kwrited so I don't know how that window went there.