As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR RETURN AND EMERGENCY DECLARED. DID AN ATB TO CLE DUE TO SMOKE FILLING UP THE CABIN. EMERGENCY WAS DECLARED AND AIRCRAFT LANDED SAFELY IN CLE. FOUND APU ENCLOSURE FILLED WITH GLYCOL. CLEANED UP GLYCOL AND RAN ENGINE. NO SMOKE FOUND.