Use this query to find where users have their post migrated but they never followed the question to the target site.
Q&A for amateur radio enthusiasts
SELECT OwnerDisplayName , Posts.Id AS [Post Link] , PostHistory.CreationDate AS [Migration Date] , Posts.CreationDate AS [Original Post Date] , SUBSTRING( PostHistory.Comment , 13 , CHARINDEX( '.' , SUBSTRING( PostHistory.Comment , 13 , LEN(PostHistory.Comment) ) ) -1 ) AS [Source Site] FROM Posts JOIN PostHistory ON posts.Id = PostHistory.PostId WHERE PostHistory.PostHistoryTypeId = 36 AND Posts.PostTypeId = 1 AND OwnerUserId IS NULL AND NOT EXISTS (SELECT 1 FROM Users u WHERE Displayname = posts.OwnerDisplayName and u.creationdate > posts.creationdate) ORDER BY 1, Posts.CreationDate DESC