Two points - FKC's car was not in the garage, and since she was just popping home for something and was with Tony L , not living at home at the time, there would have been no reason for her to have backed her car into the driveway. So it was either parked nose in on the driveway, or it was parked on the street. Walking out of their usual house entry through the garage, George would not be able to see into FKC's car trunk.
And I do not recall George ever saying he "saw" the gas cans in the trunk. He didn't walk to the trunk with her - she was already in mid temper tantrum at being "caught" with the gas cans.