It would be unusual for a one-way to revert to being a two-way IME.
Mulling on this, IMO someone parked a similar car there before noon, which was seen by School Pupil at noon and then by WJ at 12.40ish. That car then leaves. At or shortly after 2.30, 2.45ish, SJL stopped her car there, locked it and then got into her abductor's car, expecting not to be away from her own for very long. It's a lousy bit of parking, but it's only for five minutes. It is then her abductor who later returns to the car, unlocks the driver's door, puts gloves on and moves the seat back looking for something. He retrieves whatever it was he doesn't want found in the car, then goes away. He doesn't bother to steal the purse or lock the car. He's in a hurry and doesn't care if the car's stolen. In fact, it would be quite helpful if it were. He ditches the car keys, but not near the car (the drains etc were searched). Nothing conspicuous about this, which is why the BT guys noticed nothing. SJL's car and the one that was there before are similar enough that WJ fails to notice that there were in fact two cars.
This makes it possible for her to have gone to 37SR and also to have been seen in the FPR by BW - but it does require a second Fiesta.