"911, there is a suspicious vehicle."
"What's suspicious about it?"
"Uh, the license plate has some dirt on it."
"Please call 311 and stop bothering us."

Or, refuse to elaborate and hang up in hopes they do something. But they might send a bomb squad or antiterrorist team, and things may escalate not in the intended way, also may have negative consequences for the caller. (Although on a karmic level, these assholes absolutely deserve a visit from a SWAT team.)