3 people were present: Bamber, PS Bewes, and PC Myall all of whom produced witness statements. If as you say one or more individuals became alarmed about something or someone and started running it would have been a notable event and recorded as such so please find.
Please at least take the trouble to study the three policeman's statements in detail, Saxby says they came running back, Bews and Myall don't even mention the event at all.
Bews claims he called in armed police before returning to the car, Saxby says the armed police were called after returning to the car.
In three interviews I have seen Bews give, first he says it was Myall, then JB then himself who first saw the reflection / movement in the window.
So my question stands if they saw nothing why did they run??