Local police continued their search for Robin Williams Monday, three days after the movie star improvised himself off the set of the upcoming Paramount film My Three Sons. 'Robin was ad-libbing brilliantly when, all of a sudden, he lost control and began doing impressions of everything in sight,' My Three Sons director Donald Petrie said. 'By the time we summoned his handlers, he'd left the studio lot altogether.' Security guards reported seeing Williams extemporize wildly off studio property, riffing eastward down Melrose Avenue before disappearing from sight. His current whereabouts are unknown. 