AN ACR MLG CREW FAILED TO WRITE UP AN ECAM MESSAGE.

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PASSING 20000 FT IN THE CLB; WE GOT A 'BRAKES HOT' ECAM MESSAGE WITH A #3 BRAKE TEMP READING OF 650. THE CAPT DIRECTED ME TO ACCOMPLISH THE APPROPRIATE ECAM AND COCKPIT OPERATING MANUAL PROCS WHILE HE FLEW AND HANDLED ATC COMS. WE INFORMED ATC OF THE PROB AND RECEIVED CLRNC TO DSND TO 10000 FT AND SLOW TO GEAR EXTENSION SPD. WE STATED THAT WE WERE NOT IN AN EMER SIT. THE CAPT DIRECTED THE LEAD FLT ATTENDANT TO CHK THE CABIN NEAR THE GEAR AREA FOR ANY SCENT OF SMOKE. THE LEAD RETURNED TO SAY HE COULD DETECT NO SUCH ODOR. BOTH THE CAPT AND I EXPRESSED OUR SUSPICION THAT THE BRAKE TEMP HAD TO BE ERRONEOUS SINCE THERE HAD BEEN ON INDICATION OF A DRAGGING BRAKE DURING TAXI OR TKOF. WE LOWERED THE GEAR AND MONITORED THE TEMP. SHORTLY AFTER GEAR EXTENSION; THE #3 BRAKE TEMP DROPPED INSTANTANEOUSLY FROM 650 TO 85 CONFIRMING THAT THE HIGH INDICATION HAD BEEN ERRONEOUS. WE INFORMED DISPATCH OF THE SIT AND OUR FUEL LOAD AND REQUESTED CLRNC TO RESUME THE CLB. FOR THE REMAINDER OF THE FLT; THE #3 BRAKE TEMP INDICATION REMAINED NORMAL. DURING THE LNDG ROLL AND TAXI-IN AT SNA; THE #3 BRAKE INDICATION SHOWED A NORMAL RISE AND DROP PARALLEL TO THE OTHER 3 INDICATIONS. WE DETERMINED THAT THE INFLT INDICATION HAD BEEN SPURIOUS AND THAT NO PROBLEM NOW EXISTED IN THE SYS. THEREFORE; NO WRITEUP WAS PLACED IN THE MAINT LOG.