They are probably just innocent bystanders the police want to rule out and get any useful info off.
Which takes me back to a point i made a while ago ; if you were PR you woudnt take Libby to a park inhabited by local youths in minus 5 degree temperatures. Assaults happen to women in parks , rather than women being taken to a public park to assault them.
Plus , if we assume the late night search of the bench was in response to PR citing that bench as the location he and Libby hung out together in the park , if PR is guilty he woudnt tell the police the truth , he woudnt direct the police to the scene of the crime , he would direct them somewhere that woudnt incriminate himself. The police would have checked and found nothing. Ergo PR didnt take Libby to the park.
Likely the police have a timeframe between when PR is seen picking Libby up in Haworth and then returning home , but beyond this the police are having difficulty determining what happened and where it happened.
Or PR is innocent.
Really the police need to find Libby as that would fill in a few gaps in the puzzle , im surprised they arnt doing more orchestrated searches.