What was found
HARD BOIL EGG/SULFUR SMELL DURING START OF DESCENT FROM FL240, SLOWING DOWN, AND APU START FOR APPROX 1 MINUTE AT A TIME. PRESSURIZED AT THE TIME OF FIRST SMELLING ODOR. FAS DID A SWEEP THROUGH OF CABIN, NO ONE WAS EATING EGGS AND THERE WE'RE NONE IN THE OVERHEAD COMPARTMENTS. THEY WE'RE ADAMANT NO ONE WAS PASSING GAS AND NOTHING IN THE LAVS OR GARBAGE CAN FOLLOWED FIM 21-20-00-810-801-A STEPS AND COULD NOT DUPLICATE SMELL. ALSO RAN BOTH ENGINES FOR 10 MINS WITH PACK AND BLEED OPEN. WALKED UP AND DOWN THE AISLES COULD NOT DUPLICATE SMELL. TRIAD RELEASED ACFT OKAY FOR SERVICE.