can you actually open the mail following the search. Appearance in search results says it is in the global index. Not that the mail still exists. Unfortunately when things like anti virus programs delete whole folders of mail to remove a "threat" the index does not get updated.

Many thanks. I had been sidetracked by the search results. My anti-virus app (Antivurus Zap) listed the folder, location, and date deleted. I then used Time Machine to restore the file.

it is usually a good idea to exclude the Thunderbird mail and IMAP mail folders at least from anti virus scanning. They only contain text files and deleting any virus/malware in a text file is futile as it represents no risk in that state.