Craig Robinson's shirt changes in pattern when he goes from visiting the mental hospital to seeing the voodoo lady in the next scene.
The ticket agent says that the flight to Detroit is boarding in 15 minutes. They would not have enough time to get through security and to the gate.
Lisa escapes the mental hospital and runs into a group of teens drinking beer, one of whom offers a can to Lisa. As she chugs it, Lisa gets shadowed by the camera and its rigging.
The ticket agent would have informed the TSA about Charlie and Mona Lisa since they didn't seem to care about where the plane was going. This is highly unusual behavior which could be indicative of terrorism.
Mona Lisa and Charlie, who at this point are fugitives, inexplicably choose to fly, rather than take a bus out of town, which would be much cheaper and not require them to go through any security checkpoints.