Saab 9-5 has mini drums inside the rear discs for the handbrake, vw use the disk pads with a mechanical lever for the handbrake.
When there is a big freeze and whenever you have been driving in snow/slush you should park in gear and leave the handbrake off , if you do not then you risk frozen hand brake cable or mechanism that operates the brake. Automatics should be left in park, handbrake off.
Not sure that there is any option are when you have one of those electric
handbrakes.
For the OP , if your rear brakes were recently worked on then I would get them checked , just in case there is a problem related to the work done.