What was found
INFLIGHT, THE NR 1 FLIGHT ATTENDANT REPORTED THAT CABIN EMERGENCY LIGHTS WERE ALL ON. THE EMERGENCY EXIT LIGHT SWITCH ON OVERHEAD PANEL WAS IN THE ARM POSITION AND THE EMERGENCY POWER BUTTON ON PURSER PANEL WAS IN THE OFF POSITION. MAINTENANCE FOUND LOOSE PUSH BACK PIN ON CONNECTOR OF 12WL EMERGENCY POWER SUPPLY UNIT. REINSERTED AND SECURED LOOSE PIN AND ACCOMPLISHED SATISFACTORY OPERATIONAL CHECK IAW MM 31-51-00. AIRCRAFT WAS GROUNDED.