Ok, I even went out and sat in my LARGE pickup truck and tried to see how I could possibly position myself inside the vehicle (without doing the third world squat on the seat) in order to pull my shirt up and show anyone a pistol in my waistband. I couldn't get my waist level with the window sill without standing in the seat.
Maybe she had a "Show me yout T*ts" bumper sticker on her rear bumper?
Edited: Just tried it in all 3 BMWs. Less headroom and tighter knee areas. I think the guy was doing something other than trying to show her a gun. If I wanted to show someone I was carrying, I'd pull the pistol out and show it to them. Thumbs up on drive away also seems odd. I'm guessing he wanted to show her a tatoo or he just has really odd ways to try to pick up chicks.