beep noise!?

Hi. Land rover freelander 1.8 2002.When i unlock my door(s) and open the drivers door, the dash board beeps and the red LED light flashes in the instrument panel. This will continue for approx 20 seconds, or until i place my key in the ignition and turn it. This happens 9 out of 10 times, but only on the drivers door, not when the passenger or tailgate are opened? Any clues?