An apparent plot flaw (spoiler alert)

In the penultimate scene when David Jahn makes his frantic and futile attempt to convince the smug new detective on the case that the 1986 perpetrator had an accomplice at the scene, (ergo the new case isn't solved, Sinikka's murderer could still be on the loose) David goes to great pains to prove his point by referring back to the 1986 eyewitness account of the red car. The vehicle was seen leaving the crime scene traveling north with the perpetrator at the wheel, in which case the discarded headphones would have to have been tossed from the passenger side of the vehicle. But why would there have ever been any doubt in the first place that there were two men at the scene of Pia's 1986 murder? In the flashback we saw the murderer, Peer Sommer, pick up the girl's bike and throw it into the field, we also saw his accomplice, Timo Friedrich, pick up Pia's headphones. That would have resulted in two different sets of prints at the crime scene. So, why would the original detectives on the earlier case have assumed there was a lone perpetrator?