IMO Holt made the first gas stop so short because he maybe recognized a mutual friend of his/Whitneys', so he bolted out with a small amount of gas...Then minutes later, he made the 2cd gas stop for more gas and likely felt safe no one recognized him/SUV. Canteen Vending, Holt's employer, maybe had the Shell gas station vending account, and was there on its daily route. Route drivers do their deliveries pretty early in the morning. That would've freaked Holt out enough to bolt fast out of the Shell station. Also, factor in maybe the gas attendant might've noticed something odd about the SUV, maybe blood, glass, broken window and Holt sensed this, felt he better get out of the Shell station before the gas attendant got too nosy.