The only reasons that I know for a doc not to show up in Recent Documents are:
1) it was never saved; or
2) it originated as an email that you received and after you edited the doc you simply saved it instead of doing a Save As.
Normally when you save a Word doc for the first time the Save As dialog box opens. You then select a name for the doc (usually the first few words in the doc appear by default), the file type, and the location where you want to store the doc. The default file location and file type that you selected for Word appear.
None of this automatically happens when you simply save an email doc. You can't search for the doc because you don't know the file name, type, or location. If it's not in Recent Documents, it is gone forever.
EDIT: When you receive a Word doc that is an email attachment, the very first thing that you should do when you open the doc is to IMMEDIATELY do a Save As. If you wait, you will forget to do a Save As and you will do a Save when you are finished and the doc will be gone.