Seabank Road at that time of night especially in the month of November is usually relatively quiet. Of course you wouldn't grow any suspicion if you saw someone pulling up to the side of the road to where the drain is located. But once you were aware that later on that's where the murder weapon was placed then i dare say your suspicion would grow and you'd definitely be thinking "was the guy that pulled over in his land rover that night an innocent person, or was he infact the killer"? I know from a purely personal perspective that my suspicion would be getting the better of me.
Really happy that you have mentioned the Royal Marine apartments, I have brought this up in previous discussions that this could have been a likely place for the car to be parked. Its an ideal location and the gunman could easily have hidden himself in plain sight by running across the open field which would have been pitch dark by this point in order not be seen by anyone.
I don't know if there is ccttv operating on that building at all, or if there would have been at the time, but if not then it's very possible that this was the destination in which the killer used to escape, as it was on his route out of nairn.
I agree also that there is no chance that the killer parked on the double yellow lines which would have drawn unnecessary attention to himself. Something is drawing me to the links open car park or the Royal Marine apartments, i don't think we're far of the mark.
As for the last piece on your previous post, I don't think the killer took that route at all, the route which we think he used, is most likely the one he used to escape.