As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 1710 FROM TOL TO MKE. IN CRUISE, FIRE BELL SOUNDED IN COCKPIT WITH NO ASSOCIATED VISUAL WARNING, NO LIGHTS AND NO OTHER INDICATIONS. THE AIRCRAFT RETURNED TO TOL AND LANDED WITHOUT FURTHER INCIDENT. FOUND MOISTURE IN THE APU OPERATING POWER UNIT CONNECTOR AND UNIT. CLEANED AND DRIED THE UNIT AND CONNECTOR. OPERATION CHECK GOOD. AIRCRAFT RETURNED TO SERVICE.