PILOT AND MECHANIC REPORT ABOUT A B757-200 THAT DEPARTED WITH A PITOT STATIC HEAT CONTINUOUSLY ON; WHILE ON THE GROUND. LEFT AUX PITOT HEAT LIGHT WAS DEFERRED AS INOP.

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PARKED AT GATE. MECHS WERE WORKING ON PITOT HEAT PROB. PITOT HEAT STAYED ON THE GND. AFTER TROUBLESHOOTING; RESETTING CIRCUIT BREAKERS BY MECHS; MECHS COMMUNICATED WITH MAINT CTL; THEY GAVE OK FOR FLT. MECH SIGNS THE ITEM OFF. OK FOR FLT INCLUDING ETOPS #1 CHK. SINCE PITOT AIR GND LOGIC STAYED IN AIR; PITOT STAYED HOT; THEREFORE GOOD FOR FLT. IN THE AIR; THERE WAS NO EICAS MESSAGE SINCE PITOT HEAT FUNCTIONED NORMAL FOR FLT. SUPPLEMENTAL INFO FROM ACN 800131: ACFT X WAS HERE AT ZZZ WITH AN OPEN MEL. THE MEL AUTH NUMBER WAS FOR AN INOP L AUX PITOT HEAT LIGHT; PROBE HEAT WORKS. DURING LOGBOOK REVIEW; THERE WAS A YELLOW PLACARD WITH CHK BEFORE DEP BOX CHKED. I WAS THEN SENT TO ACCOMPLISH THE MEL PROCS MANUAL. PAX WERE ON BOARD. WHILE LOOKING AT THE OVERHEAD ANNUNCIATOR PANEL; I NOTICED THE R AUX PITOT HEAT AMBER LIGHT CAP WAS PUSHED ALL THE WAY IN AND LIGHT CAP BROKE. I ORDERED THE ANNUNCIATOR PANEL. PART WAS LOCATED AT THE HANGAR. IT TOOK A LITTLE WHILE TO GET THE PART. UPON REVIEWING THE MEL; NO YELLOW PLACARD WAS REQUIRED. LEFT THE WHITE PLACARD IN PLACE (LOGBOOK). I REPLACED THE ANNUNCIATOR PANEL AND MADE APPROPRIATE MAINT AND BAL ENTRY. HOWEVER; YESTERDAY IT WAS BROUGHT TO MY ATTN THE POSSIBILITY FOR THE PROBE HEAT TO BE CONTINUOUSLY ON WITH ACFT ON GND. AFTER REVIEWING THE PROBE HEAT SYS; TO ME IT SEEMS IT IS POSSIBLE TO MISINTERP THE ABOVE POSSIBILITY WITH AN INOP PROBE HEAT LIGHT AND STILL PASS THE MEL PROCS MANUAL. I SUGGEST MEL CLARIFICATION TO DISCRIMINATE PROBE HEAT AMBER LIGHT PROB (LIGHT OUT ON GND) FOR AN INOP HEAT LIGHT AND A PROBE HEAT WHICH COULD POSSIBLY BE CONTINUOUSLY ON ON THE GND. I HAVE REVIEWED COMPUTER BASED TRAINING FOR YELLOW PLACARD TRAINING TO MAKE SURE I REVIEW MEL FOR CORRECT PLACARDING REF.