This issue has been reported in multiple forums elsewhere. It appears that the switch in the door is faulty, however many responses have suggested that because its a flaw in the design of the part, the replacement part may fail as well.
Suggested fixes (if you're outside the warranty window) include using a sensor cleaner (a lot of it) in the sensor contact area but that made mine appear even more so. That being said, I have found that if you acknowledge the alarm by pressing "OK", then opening and re-closing the door, it seems to disappear (until next time). I used the sensor cleaner over a week ago and this fix seems to be working.