For the last few weeks, I've noticed weird red dots/lines or similar marks on my Kubuntu screen. I'd attributed them to a graphics issue that I'd recently sorted out, particularly because I was mainly noticing these marks when resuming the system from sleep mode. Turns out I was wrong. Very wrong.
You see, I've become a fan of KDE's Activities feature and frequently switch between activities. I do this using the Meta (Windows key) + TAB and Meta + Shift + TAB. It so happens that a hitherto unknown KWin feature named Mouse Mark uses a similar combination. Meta + Shift in combination with mouse movement allows you to draw on the screen, a feature which I suppose would be handy during presentations and such. Consequently, when I switched between activities using a similar key combination and incidentally nudged the mouse a little, a small mysterious red mark appeared on the screen.
Well, mystery solved :)
The Mouse mark feature is part of KDE's desktop effects. To disable it, go to System Settings, Desktop Effects, and choose the All Effects tab. Uncheck Mouse Mark and click on Apply.
Now that I know that this is a feature, I'm leaving it switched on. To simply remove the most recent mark, I now press Meta + Shift + F12. To remove all marks, i.e., to clear the screen, I press Meta + Shift + F11.
Hope this helps :)