As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF FROM DTW, A BURNING RUBBER/OIL ODOR FILLED AIRCRAFT. FLIGHT RETURNED TO DTW AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AIRCRAFT INSIDE AND OUT AND COULD NOT LOCATE SOURCE OF ODOR. AIRCRAFT WAS APPROVED FOR FURTHER SERVICE.