9.10 - Nicola was seen in the top field (at a distance) by someone who recognised her, their dogs interacted. Her phone was linked to works conference call on mute. Dog was running free (presumably Nicola had the harness & lead over her shoulder).
From then onwards there are no witness sightings.
IF, IF someone known to Nicola then entered either the top or lower field rushed up to her, told her, say, there was a problem at school with the children, IMO she may have called the dog, but if Willow didn't immediately respond (rushing off chasing squirrels) it would be understandable if Nicola urgently left the scene & went with the familiar person to attend to her children.
It would not be unheard of then for such predator to return to place her phone, lead & harness near the river. Willow, the dog, now knowing where Nicola was, may have made her own way there as it was a spot she was familiar with.