What was found
AFTER COMPLETION OF FLIGHT 2146 (MRF TO PDX) AIRCRAFT TAXIED TO GATE, PASSENGERS DEPLANED NORMALLY, GROUND SERVICE AGENT HOOKED UP GPU PLUG TO THE AIRCRAFT, FLIGHT ATTENDANT NOTICED WHAT APPEARED TO BE A FIRE AROUND GPU PLUG AREA ON AIRCRAFT SIDE. FLIGHT ATTENDANT UTILIZED ON BOARD FIRE EXTINGUISHERS TO EXTINGUISH FIRE. AIRCRAFT WAS RELEASED TO MAINTENANCE FOR INSPECTION. REPAIRED IAW DHC-8 TASK 23-50-00-710-801 AND 24-41-00-710-801. OPEATIONAL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE.