Hi Peter,
Many thanks for providing the solution to this annoyance. Just for your feedback:
I decided to try the file search before upgrading, so I could repeat it if the problem recurred. Once you told me how to get into the Contents folder, I was able to do a manual search while Terminal was searching simultaneously. That was fortunate, as there was no file called BorisFX_autorecovery.bfx, and Terminal wouldn’t have found it. The file was actually called BorisFX_autorecovery (without the .bfx). It was located in the folder Applications/Final Cut Pro.app/Contents/MacOS (not very deep down at all). On removing it and starting the BFX plugin, the message didn’t reappear.
I will now be updating to 9.2 anyway. Thanks again, and hoping this may be of help to you and other users.