As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CAPTAIN REPORTED A SMELL WHEN THEY STARTED THE APU. FLIGHT ATTENDANT SAID IT WAS THE DIRTY SOCK SMELL. THE CAPTAIN SAID IT WAS MORE OF A CHEMICAL SMELL. CABIN CREW REPORTED HEADACHE SYMPTOMS. ACCOMPLISHED THE CABIN AIR QUALITY TROUBLESHOOTING TASK CARD 26L0021001, NO ODOR OR SMELL WAS OBSERVED IN THE CABIN & NO HISTORY OF SIMILAR EVENTS IN THE PREVIOUS 90 DAYS.