Question: Wouldn't the mall staff have removed any gas from the car parked inside the mall?
Question: When the group first meet up at the dock, just as they are about to set off in the helicopter, when Roger closes the door, he then looks out of the window - eventually he gives a smiling nod. This could simply be a nod to the other group but it seems as if it may have been unintentional possibly as if he thought that the camera had stopped rolling. Curious to see if anyone could clarify it. Maybe I'm just looking too much into the film.
Answer: One of the cops that was with Joe Plato (how much you have seen of them will depend on which version of the movie you, but I HIGHLY recommend the fan made/edited 'Extended Mall Hours' cut on YouTube) asked them for cigarettes. You'll notice that Roger and Fran lit up cigarettes as they were taking off, that's what Roger was laughing at.
Answer: Not necessarily, there are many instances where cars are parked inside of malls or even stadiums for promotional purposes. It is a lot easier to have gas in the car if it needs to be moved or replaced for any reason.
Invader_Gir