What was found
APU WARNING ANNUNCIATOR PANEL ILLUMINATED AFTER TAKE-OFF. (APU WAS OFF AT THIS TIME) CREW FOLLOWED EMERGENCY CHECK LIST AND DISCHARGED THE FIRE EXTINGUISHER AND RETURNED TO FIELD. UPON INSPECTION BY MECHANICS, FOUND NO SIGNS OF ACTUAL FIRE, IT APPEARED TO BE INDICATION ONLY. MAINTENANCE REPLACED FIRE EXTINGUISHER BOTTLE AND CARTRIDGE WITH SERVICEABLE UNITS. GROUND OPS CHECK OF FIRE WARNING SYSTEM SATISFACTORY. GROUND CREW OPERATED THE APU FOR 30 MINUTES AND THEY WERE UNABLE TO DUPLICATE THE PROBLEM WITH THE APU ON OR OFF.