Strange effect, "collection - marked pictures"

I found over 500 mostly empty folders in my collections folder, which i can't explain. Looking at their names, they seem to refer to some photo folders i have. I have no idea, how or when i did create them. Is this normal?

Is Preferences / Viewer / Behavior / Save marked images to a file collection... on and did you mark some images in the viewer?

Yes, i do that very often, everytime i'm selecting stuff from my photo sessions, and also yes, that option was on. I will see later, if i need it to turn back on to maintain my workflow. I thought that those results shown in the collection were temporary ones.