From one of the local Glasgow forums - Speculation, but may give a picture of what it was up to prior to the accident.
'Will be a miracle of no one dies. Can't help but think it has run out of fuel trying to get back to SECC. A woman from Govanhill said it had been hovering over head with spotlight on railway line for quite a long time before incident.'
'I saw something on twitter during the day that a person was hit by a train on the Glasgow central line today, so it makes sense that the helicopter may have been doing something related to that incident.'
The junction itself is well served for CCTV but the police seem to now be appealing for video to be sent in which suggest it may have missed the incident.
The Sun Editor was probably on the Qparks car park roof. From memory that has no view to the South (Clyde) and limited views to the East (Cathedral blocks view towards Clutha) and West (Tenements) although it's slightly better to the East if he was at that end of roof. That suggests he probably observed it coming in from the North or North-East which implies a path across the large King street car park.