Hi Stella,
no, there is no evidence that Dirk was taken away by car. I think the parents only started thinking along those lines because Dirk vanished without a trace. This case is very close to my heart and I read the book (in German) and also watched some German interviews with police officials. According to what I read, nobody really entertained the sinkhole theory although that thought crossed my mind as well, especially as sinkholes are mentioned in the book. The soil was frozen solid, can sinkholes happen when the soil is frozen solid? My guess is they can't and this is why that theory was not pursued. I think the assumption was that he fell into a tiny local river and drowned and was swept away. There was a little stream and Dirk and his sister played by this stream when their parents went back to the parking lot. However, that stream was frozen solid. A short distance away there was a little river, probably only 3-4 feet wide that was not frozen. When the search crews could not find him, the assumption was that he fell into this river, drowned and that the body was then carried away in the water. But apparently a little further downstream there was a metal grid/grate and the body would have been held up and found there. Nothing was ever found, not even a shoe or glove. According to police and search crew officials, it is extremely unlikely for someone even that small to vanish without a trace in that sort of topography.
So, the fact that there was another car at the scene which left at exactly the time Dirk went missing raises questions. Of course the couple in the car might have nothing to do with what happened and this might be a tragic case of a drowning accident with no body found. However, the couple's identity is unknown and Dirk's parents were persecuted quite viciously by the secret police when they started asking questions. They could have been persecuted because the East German state simply didn't like it that they asked questions or they could have been persecuted because there is more to the story.That's the bare bones of the story.