That's one of many questions I had. I can only assume, since they didn't show it, that he somehow carried her out through a quiet entrance into the car park, where he put her into his car. We only saw CCTV of him getting into the lift to go up to the tenth floor, not coming down.
I also was amazed that, of all the times Sarah visited the Larsens' house, did she only notice the broken rear light right at the end. Terry was obviously still using the car, but it hadn't been noticed until then.
They also didn't show us who put the picture on Linden's fridge.