After having their baggage searched for more than three hours upon arrival, the Baltimore Symphony Orchestra and music director David Zinman were given a standing ovation in Moscow after their first performance Wednesday. The orchestra--the first American orchestra to play in the Soviet Union in more than a decade--replied to the positive response with five encores, including an orchestral arrangement of the classic rock tune "Twist and Shout." The "standing O" soothed the tempers of some of the musicians, who were angered when their luggage was retained Tuesday at Moscow Airport Customs and some of it apparently searched in secret. "When the bags came through, most of them had obviously been opened. Some things were damaged," one musician said.