Could be wrong but unless they have evidence showing a clear struggle - ie. Libby forcibly put into car, then evidence of Libby being in the car (if it existed - we don’t know what forensics found) wouldn’t be enough to proceed with abduction charges. Libby could have entered car willingly and P. could be saying that he dropped her off somewhere and has no idea where she went after that.
I think this is exactly the situation the police are in so far.