IIRC, they traced all the vehicles and spoke to the owners, one of whom had no legitimate reason to be there, according to NU:
"She said four vehicles had entered the area where her son was last seen and said she believed one of them did not have a legitimate reason to be there."
https://www.theguardian.com/uk-news...rrie-mckeague-says-he-was-social-hand-grenade
That suggests the possibility that the driver has something to hide, but could be entirely innocent, or at least irrelevant.
Following on from what JustJo said, I think SP have been accepting what people tell them too easily and too often - as shown by the wrong bin weight, which took ~5 months to come to light.
On the other hand, they apparently dismissed the sighting at Fiveways because there was too little detail, they say.