Sid McCausland will retire as chief executive officer of the $1.4 billion San Mateo County Employees' Retirement Association, Redwood Shores, Calif., next March 31. A search for a successor is expected to begin shortly; no headhunter will be used. Mr. McCausland is best known for his unsuccessful fight against a $400 million raid on CalPERS, where he was executive director. That goal-line stand, combined with alienating state Treasurer Jesse Unruh, cost the feisty executive director his job in 1986. "He was a Secret Service guy jumping in front of the bullet," said M. Barton Waring, managing director, Barclays Global Investors, which manages money for the San Mateo fund.