In MS Outlook if a Sent email disappears and is not in Deleted Items, where could it have gone?

Check with the recipient. Perhaps they have it now.

when you delete Sent items, they are gone permanently.

why you would ever WANT to delete anything in the Sent folder (other than saving space) i'll never know, as it is useful to ensure you actually sent something to someone, and to prove it later...

Is it in the Sent folder?  How about the Outbox?