Car: VW Polo 1.9 TDI 100 Sport 2002

Problem:

When the driver door is unlocked with the wireless keyfob, the central locking unlocks the car and deactivates the alarm. No problem there

Open the door and first thing noticably wrong is the light on the bottom right inside door panel is not lit.
Now with the door open and waiting roughly 20 secs, the door locks and the LED starts blinking by the lock indicator. Few seconds later the alarm goes off!

To me this sounds like the feature if you unlock the car then walk away, it relocks itself automatically, so for some reason the car is not registering the door being open.

If the keyfob is double pressed to unlock everything, the mentioned lamp illuminates on the open driver door as soon as the passenger door is opened.

Is this a common problem and what is the likely cause?
Fortunately I've still got warranty so it should be fixed FOC.

Cheers

Owen