Mendeley shows files I did not import

Hi, When I check the "All Documents" tab in Mendeley desktop, it shows lots of files that I did not import. How did they got imported to Mendeley database? and How do I remove them with deleting the original files in my hard drive? I did not set up any Watch option and I did not use the Tidy up option as well. I only drop and drag files from my computer folders to Mendeley desktop folders. Regards