What was found
UNCOMMANDED TURN, DURING CRUISE PHASE OF FLIGHT WITH LNAV & VNAV SELECTED, THE ACFT STARTED A RANDOM TURN TO THE LEFT. SWITCHED TO HEADING SELECT TO TURN BACK TOWARDS PROPER COURSE THEN RE-ENGAGED LNAV WHERE UPON THE ACFT NAVIGATED NORMALLY. THE ONLY PILOT INPUTS BEFORE THE WAYWARD TURN WAS AN UPDATE OF THE WINDS. INTERROGATED MAT & FOUND NO REPORTED MSGS WITH REF FLEET TEAM DIGEST 777-FTD-34-10008. EVENTS SUCH AS THIS ARE CURRENTLY UNDER INVESTIGATION IAW EO REQUEST. PERFORMED FMC BITE DATA DOWNLOAD, FMCS BITE, FMLBPQL, FMLBPQR IAW AMM 31-35-00-2. HAS NOT YET DISCLOSED FINAL ACTION DUE TO ONGOING ANALYSIS. PERFORMED FLIGHT MANAGEMENT COMPUTING SYS ADJUSTMENT TEST IAW AMM 34-61-00-5 WITH ALL TESTS PASSED. ACFT OK TO CONTINUE SERVICE.