In all newer Saabs the actual key is detachable from the electronic key fob that activates the ignition.
Technically the hidden key is so you can unlock your car if your remote stops working. But, This can be used to keep the car warm if you are going into a grocery store or something!
Just pull out the metal key, turn on the car with the fob, lock all doors except the driver’s, get out and lock from the outside.
Warning: In some countries and states it’s illegal to idle the car with the engine on. Check the laws in your country: