I've got the same with a Buick and according to the service guys, it's a £300 fix, as they reckon it's a duff sensor.
They suggested whacking the car into Park, and wiggling the gearstick around to try and 'unstick' the sensor. Plus it also might be related to the cold too.
Here's a wild card. I've had this problem on my VW automatic. If the button on the side of the gear selector is stuck in, you can't remove the ignition key. In the dark it can be very hard to spot that the button is stuck in.
All done...Went out a couple fo times last night. Wouldn't budge. Went out this morning and kept gear selector switch in and out it popped. You cant beat German engineering.