Evidently many of the Washington State Police cars are equipped with rather sensitive radiation detectors as a homeland security precaution. Mr. Apatow had been through some medical tests before driving to work that involved injecting a small amount of radioactive material into his blood in order to monitor his circulation. It was this that set off the equipment in the nearby police car as he was driving.
His physician had given him a document that stated that he had been given a radio-isotope. He was sent on his way without incident.
One wonders what went through his mind. The doctor had already advised him to avoid close contact with family members until the isotope had cleared his system, now this. One could be forgiven for being a bit apprehensive.
Be seeing you.