it could be anything, most probably an intermittent problem with a sensor. You wont know for certain unless you get it on vagcom and it will tell you exactly then. Any decent vw garage will do this free of charge for you.
Where do you live then?
There aren't any decent garages round my way that will scan a car for free 
To the OP, if you haven't got access to VAG-com, unplug each sensor one at a time, test each one for resistance with a multimeter.
Make sure the ignition is off when you do this.
If any of the sensors read open circuit, then it is faulty.
The front sensor plugs are behind the front brake discs, the rear plugs are under the rear seat.
Hope that helps 
SOLO VW in Burton. local(ish) vw specialist. they wont charge me for hooking up the car and reading error codes in my lunch time etc.
You are very fortunate then. I've never found a garage that will do it for free.
Not that it matters as i've got my own
