My 2012 MacBook Mini runs macOS Catalina. I was surprised to find my trash can full of documents, videos, pictures, and other items that I had not removed when I went to recover a file that I had just deleted. The variety of files makes it quite unlikely that I could have accidentally deleted them. About a dozen of them were still in their original positions when I checked them, albeit some of the trash can names had changed. Is my computer throwing things in the trash on its own? I am grateful for your help.
Perhaps, you have connected the flash drive to the Mac and deleted unwanted items on the flash drive, but you haven't emptied the Trash can, then directly ejected the flash drive or taken out it from the Mac.
If this is the case, then the deleted items are still using the space on your flash drive, then you need to reconnect the flash drive to your Mac and empty the Trash.