A B727-200 ON TKOF EXPERIENCED THE LOSS OF A UPPER SURFACE CLOSURE PANEL ON THE L HORIZ STABILIZER DUE TO NOT BEING PROPERLY SECURED AFTER MAINT.

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RECEIVED PHONE CALL REQUESTING LARGE LIFT TRUCK AND MAINT PERSONNEL TO REMOVE AND REPLACE L ELEVATOR POS SENSOR AND NULL; PER MAINT MANUAL; ON ABC ACR B727-200. MYSELF AND ANOTHER XXX MECH WENT TO ABC ACR'S OFFICE TO RECEIVE INSTRUCTIONS AND TO PULL MAINT MANUAL REF FOR REMOVE/REPLACE ABOVE SENSOR. WHILE PERFORMING ABOVE; TOP ELEVATOR PANEL WAS REMOVED AND TO MY KNOWLEDGE WAS REPLACED. AFTER REMOVING AND REPLACING SENSOR; WE STILL NEEDED TO NULL SAME. WE CAME BACK TO XXX OFFICE TO GET TEST EQUIP FOR NULLING OF SENSOR; IT WAS SHIFT CHANGE AND MY PARTNER DID NOT WISH TO STAY FOR OVER-TIME. I RETURNED TO ACFT AND NULLED SENSOR; CLOSED LOWER PANEL; RETURNED TO ABC ACR'S OFFICE. I SIGNED ITEM OFF AND RETURNED TO XXX OFFICE. THE NEXT MORNING ACFT WAS ON THE TKOF ROLL AND THE L UPPER ELEVATOR PANEL BLEW OFF AND WAS RPTED BY ANOTHER ACFT. THE ABC ACR B727 FLEW TO ZZZ AND WAS CHKED AND FOUND PANEL MISSING. ACFT WAS INSPECTED AND WAS FERRIED BACK TO YYY; WHERE PANEL WAS REPLACED. TO THE BEST OF MY KNOWLEDGE PANEL WAS INSTALLED AND SECURED.