What was found
APU FIRE FAIL CAUTION MESSAGE ENROUTE CAME ON AND OFF TWICE FOR ABOUT A SECOND EACH TIME. COMPLETED THE QRH FOR APU FIRE FAIL CAUTION. 5 MINUTES LATER APU FIRE WARNING MESSAGE CAME ON AND OFF FOR ABOUT SECOND, THEN CAME ON AND STAYED ON ABOUT A MINUTE LATER. PERFORMED THE QRC/QRH PROCEDURES FOR AN APU FIRE WARNING WHICH LED US TO EMPTY THE HALON BOTTLE INTO THE APU COMPARTMENT. APU FIRE WARNING WENT OFF AFTER HALON BOTTLE WAS RELEASED. (FLT#3587) CLOSED FOUND NO EVIDENCE OF FIRE IN APU ENCLOSURE DEFERRED BOTH APU FIRE DETECTION LOOPS IAW MEL. NO MX ACTION REQUIRED PLACARD INSTALLED.