This idea came up while I was discussing a wrongly migrated question with Programmers moderator @ChrisF. Chris notices the following:
There's still a significant proportion who use the migration option instead of just voting to close as off topic.
We thought it might be helpful to at least let people know when they've migrated a question inappropriately. Can we get an automatic flag on questions that are migrated to other sites and then closed as off-topic? Or better yet, can Community automatically post a @comment to each user who voted to migrate (with a link to the target site's FAQ)? Over time this should help raise enough awareness to slow down the number of questions that get migrated to the wrong sites.