What was found
AIRCRAFT WAS GROUNDED: AIRCRAFT RETURNED TO ATL DUE TO AFTER LANDING GEAR RETRACTION, LANDING GEAR DOORS NOT CLOSED MESSAGE ON ECAM. FLIGHT CREW CYCLED GEAR AS PER PROCEDURE LEFT GEAR #1 INDICATED AMBER AND OPEN. STAYED BELOW ALL AIRSPEED RESTRICTIONS DURING FLIGHT. MAINTENANCE FOUND PROXIMITY SENSOR 29GA TARGET BOTTOM BOLT TAB WASHER UNLOCKED ALLOWING THE BOLT TO BACK OUT. MAINTENANCE INSTALLED BOLT AND LOCKED THE TAB WASHER ON THE TARGET PER CMM 32-31-33. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED.