Well, as I read it...
You seem to be suggesting that Mags should have known she was reversing before her reversing lights lights came on, since they weren't on when he rode to the central reservation (and the parking car was not clearly visble then either, I think.)
'watching ahead' presumably means ahead in time, since she was showing no sign of reversing when he moved behind her.
Both these imply to me that you think Mags should have expected the car to suddenly reverse direction, when t was showing no sign of doing so and therefore not to have gone behind it.
Which I guess was what Crackle though too, and I know whose opinion and logic I value more...