I've had that happen with search terms. For example, if I search for "home" (with or without other search terms), then "home" will be highlighted as you describe in any post for long after I've closed the search window. It's annoying. I believe that deleting FlyerTalk cookies will fix the issue (although you will need to login again). Alternatively, search for some other term that appears less frequently. The new term will be highlighted when it is in a post, but at least it will appear less frequently.