When Lila picks up her mail, the flag is up on her mailbox. Having the flag up means the postal carrier hasn't picked up/delivered for the day.
A Humvee is diesel, the sound it makes when it dies on the road is the sound a gasoline engine would make. A diesel would not make that sound.
In the exterior shot of the motor pool just before viewers are introduced to Carter and Angie, the flag on the building is a 48-star flag, last used as an official flag in 1959.