For Plasma the desktop configuration file is saved at `~/.config/plasma-org.kde.plasma.desktop-appletsrc`. There you can see all widgets, which are placed somewhere on your desktop.
**How do you know which of these widgets are off-screen?**
Some configurations show a `lastScreen=-1` property. If it's `-1`, it's obviously off-screen. Another way to tell is to look at other information. For example a task manager, here you can look at which application are pinned to it. If there are applications pinned to it, you don't have pinned on you currently visible bars, it's probably off-screen.
**Notice**
When deleting, always delete all parts of a widget. The header of a widget looks like this: `[Containments][1016][General]`. If you decide, that this widget is off-screen and delete it, make sure to delete all `containments` which have the same id.
Just in case, you should make a backup of the file and then start messing around with hit.