Hi, same problem: Office 365 for mac, and I routinely went to the caches/temp items folder to find my files (over past few years, but then changed computers/office recently). I spent an hour 'digging' based on your good advice, but to no avail. I even searched on any folder updated to the time I had saved the doc, and nothing.
I was, however, able to find the folder with the same name as the one I could see but not access under 'Recent files', but searching a part of the name therein was unsuccessful. I assume this may because the file is now 'invisible'. How to search among invisible files? Any further advice?
When you go to the File > Save as. Menu, you'll be shown the temporary folder location Outlook is currently using for the attachments and will likely find your missing file there (note: Outlook. If, like me, you’re a multi-platform user, you’re probably quite familiar with locating temporary files on Windows. But on a mac, there are LOADS of hidden folders, including “TemporaryItems”, which is where your Outlook Temp Folder lives.