|If your manager creeps into the office, it is not surprising to find that you get the creeps. |
Your first option is simply to tell him that you tend to be deeply engrossed in your work, and as a result you do not always hear him enter. Then ask him if he would let you know when he enters so that you can talk with him immediately and not waste any of his time.
Another option is to reconfigure your office. Depending upon the layout, perhaps you can move your computer to allow you to see when someone enters your office.
If things cannot be moved, some might suggest a ringer mechanism that goes off when people enter your office, but that can become bothersome and distracting for you and those who work nearby, especially if you have frequent visitors. A less obvious step is to have a mirror on your desk that serves the same purpose as the rearview mirror on your car. In all respects, it is important to know if your manager is behind you.