As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR RETURN TO DEPARTURE FIELD {KEWR} AFTER REPORTS OF A BURNING PLASTICS/ ELECTRICAL NATURE IN THE AIRCRAFT CABIN. PERFORMED FIM TASK 21-00-00-810-806. NO DEFECTS NOTED. ALSO PERFORMED HIGH POWER RUN WITH & WITHOUT PACKS, RAN ALL GALLEY EQUIPMENT, NO SMELL APPARENT. SUSPECT NR 1 ENGINE BOROSCOPE PLUG ANTI-SEIZE COMPOUND & DISSIPATED BEFORE LANDING. OK TO CONTINUE.