As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SMOKE IN CABIN DURING PREFLIGHT WHILE APU WAS RUNNING. FIRE DEPARTMENT INSPECTED WITH HEAT SEAKING EQUIPMENT AND DID NOT FIND ANY "HOT SPOTS" LESS THAN 20 MINS AFTER APU SHUT DOWN. FOUND LARGE BIRD NEST IN APU EXHAUST EXTENSION TUBE. REMOVED TUBE, CLEANED OUT TUBE, AND INSTALLED. REMOVED WATER SEPARATOR SOCK, INSPECTED AND CLEANED. INSTALLED SOCK. OPS AND LEAK CHECKED. ALL CHECKS GOOD. WORK DONE IAW MM 21-51-00.