19. 1. Ascertain current state of danger for the pet; 2. Smash window.
I find that smashing the driver's side front window to be the biggest inconvenience for them.
You'll want to be able to secure them with a leash or something. Having a rescue, I can leave a note telling them how to contact me to get their dog back. They should have cooled down by the time they come for him/her (hence alleviating any revenge on the pet for the broken window, because you know...it's the animal's fault.