Where are the file's calibre is adding, are they always in same place - e.g. your Desktop or Downloads folder? If so, move them somewhere else - e.g. a "Work" folder on your C: drive, and then start calibre.
Have you installed the JobSpy plugin? It has a per library tweak for AutoAdd:
Another possibility is the Windows Send to menu:
The
_calibre item I've boxed is a shortcut to calibre.exe, if I clicked it as shown, the
Inside the Carbon Cult PDF would be added to the current library. Send to is the primary mechanism by which I add books.
Like most engineers (ChemEng in my case) I don't like inexplicable phenomena. He might move in mysterious ways but the things He's alleged to have created don't do that… not even nuts that fall off drawer handles.
BR