In some cases, user just open up a temporary document downloaded from web browser. In Firefox, usually this happens when downloading a file then user select "Open With: LibreOffice _MODULEHERE_". This document then usually placed in temporary directory or folder (/tmp/mozilla0_username in case of GNU/Linux user).
A day after that with assumption the machine has been restarted, the file then lost as expected from a temporary file handling.
LibreOffice should automatically delete this file in the recent file as the file has been deleted, or set it as grayed out to tell user that the file has been opened.
*** This bug has been marked as a duplicate of bug 101302 ***