No one at the opening of the Richard Nixon library last week mentioned the missing 18 1/2 minutes on the infamous Watergate tape. But some guest were talking about former First Lady Nancy Reagan's 16-minute absence from a luncheon given by the Nixons after the ceremony. During the meal, sources say Mrs. Reagan left the table to chat with a former White House staffer. After about 15 minutes, she returned to her seat next to Richard Nixon. "The rudest thing I've ever witnessed," said one onlooker. "It was a lovely lunch," said a Reagan spokesman. "We don't comment on mischievous gossip."