What was found
WHILE LEVELING AT FL260, FO FLYING FROM RIGHT SEAT, WE NOTED THE LEFT SIDE ALTIMETER WAS READING APPROXIMATELY 300' LOW AND VSI SHOWED A DESCENT. FD MODE OFF CAS MESSAGE CAME ON AND AUTOPILOT FAILED. WE VERIFIED THAT THE RIGHT SIDE SYSTEM WAS CORRECT AND DIVERTED. AIRCRAFT SAT IN RAIN OVERNIGHT AND ENTIRE FLIGHT WAS IN LIGHT TO MODERATE PRECIP. OAT AT FL260 WAS AROUND -20 TO -30 C. PERFORMED AIR DATA TEST, PER CE-680 AMM 34-11-00 AND FOUND NO LEAKS AND THAT THE PITOT STATIC SYSTEM IS WORKING PROPERLY. NO FURTHER DEFECTS NOTED AT THIS TIME.