In “Need option “belongs to CodeReview” in “close because off-topic” button”, it was explained that
If you see a question that you think would be a better fit for another site not on the migration list flag it for moderator attention explaining where you think it should go.
I didn’t think of that. I think most people won’t think of that. The “close as off-topic” dialog should contain some text to this effect to highlight the possibility.