My alarm system [not with Comcast] had a habit of showing the front door open as well. Trouble was, according to the alarm monitor it was! The door sensor did not match up correctly with the alarm sensor unless you stood there and pushed on the door. We had to have the builders come back in and reseal the door so it would match up.
Not saying this is your problem, though. You may need to call Comcast and have a tech come out.