There is usually always a member of staff by the gates in case of malfunction. I would have presumed that as he wasn't in a vehicle,having crashed it, they'd know he'd have to remain on foot or get public transport. I would then imagine a high alert was passed directly to all staff working in that sphere. Easy to do these days with mobiles, radios etcetera.
Back to the bridge CCTV, it's recorded him there on the bridge even if it's after the event recording. If it's constantly recording then surely it would show him either jumping, going down the bank or leaving the area.