My thoughts..
Ok so something happens and NB leaves...
Willow is left running around in field with lead and harness on.
Phone is on floor or bench.
First woman finds dog but has appointment so has to leave. Ties Willow to bench. Rings and raises alarm that dog is there.
Woman leaves. Dog gets out of harness leaving harness at bench. Possibly knocking phone on floor?
Couple arrive through gate woman goes to get dog that is now "agitated", man notices and picks up phone. Not tech savy but sees her picture on screen saver. Puts phone on bench. Phone calls are made. Police arrive finding phone and harness on bench. Looks like items just left there so assume she might of gone in river due to lack of clarity at the time. I also think something sadly bad has happened here