in the back of mind
in the back of (one's) mind
In one's thoughts, but with little or no conscious or active consideration or reflection. Despite my success at work, there's always this worry in the back of my mind that I'm doing something wrong. You don't need to do anything about it now—just keep it in the back of your mind going forward.
*in the back of someone's mind
Fig. remembered by someone, but not very important; vaguely remembered by someone. (*Typically: have something ~; keep something ~; leave something ~; put something ~; remain ~; Stay ~.) You should put this problem in the back of your mind and concentrate on other things.