As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WEATHER RADAR IN AUTO WX+T AND WX MODES IS GENERATING A FALSE WEATHER RETURN ON BOTH ND'S AT A 070 BEARING APPROX 5 NM FROM AIRCRAFT. FALSE RETURN IS APPROX 2 MILES IN DIAMETER, MOSTLY GREEN WITH SOME MAGENTA. PROBLEM DISAPPEARED DURING DESCENT FROM CRUISE FLIGHT. GROUNDED - YES. PERFORMED B737-800 FIM 34-43 TASK 801, BITE PROC. CHECKED GOOD. NO FAULT NOTED.