I was fortunate in being off duty but believe the part of the road the vehicle was left on was actually the A23 which isn't BAA property, despite the fact the South terminal is built over it.
The police do regularly patrol the surrounding roads so I guess it would have been picked up fairly soon. None of the reports say how long it was there...
It's inappropriate to comment on the CCTV coverage of the airport but again I am sure it would only have been a short time before it was picked up. It's simply isn't physically possible to watch every image from every camera at the same time.