I thought it was probably someone seeing it leave the road and phoning in, but the car then coming to rest out of sight of the road, explaining why they didn’t get any more calls about it, which IME is what normally happens. Just my guess though.
EDIT Looks like the wrong guess
We know that just prior to 11.30am on Sunday 5th July 2015 a member of the public contacted Police Scotland via the 101 system to report that they could see a vehicle down an embankment near the M9 slip road at Bannockburn.
All callers to 101 receive an electronic options menu. This call was answered within 6 seconds following that message by an experienced officer and the relevant details were given by the caller.
For reasons yet to be established this call was not entered onto our police systems and not actioned out to operational teams in the Stirling area to respond and trace the vehicle.
Shortly before 10am on 8th July 2015 a second call was made to Police Scotland via the 101 system. This call was answered, recorded and allocated to operational teams who located Mr Yuill and Ms Bell within their car.
Chief Constable’s statement