Walk past the car and photograph the driver. They really love that.
I assume you're being sarcastic, but they might actually "love that" if their objective at that point of the investigation is to alarm you and see what you do. It's an acknowledgment that they have indeed alarmed you.
I can actually do them a favour. That agent doesnt like sitting there all day. He was assinged to intimidate. Mission acomplished. By photographing him you acknowledge the message and he can be assigned to sonething else. Moving the nonverbal conversation along might actually trigger them to make a formal, polite, approach.
A lone FBI agent wont be happy, but he wont break cover be reacting. He will just drive away.
I assume you're being sarcastic, but they might actually "love that" if their objective at that point of the investigation is to alarm you and see what you do. It's an acknowledgment that they have indeed alarmed you.