[Rain Man]

I'm no law enforcement agent, but it does seem to me like the preferred approach would be:

1. Approach the girls, say hi.
2. Show your badge and say, "It looks like you've got some beer."
3. Look at water, say "Never mind. Have a nice day."

I'm not sure that hood-jumping and gun-drawing were necessary unless she got belligerent at the three steps above and got in her car or started it after Step 3.