Why did Mattie worry about being stopped by police on the drive home from the picnic spot?

"We went back at that time of evening when it’s dusky but the headlights don’t really help yet. Mattie said she was stone-blind this time of night, so Estevan drove. “Be careful now,” she warned him as the three of them climbed into the cab. “The last thing we need is to get stopped.”

They shouldn't have all been in the cab of the car.

also, Estevan is an illegal immigrant.