I suppose in general, if you are not at fault in causing that malfunction, then there is no reason not to report it. If you do report, but infact were not required too, the people at the NTSB would probably let you know anyway. Better safe than sorry, i guess.
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**edit** I just opened up the 'fat book' to check it out. 830.5 (a)(4) only referes to actual inflight fire. So by going by the book, no you would not have to notify it if you don't notice fire 'right away' .
However, you are obviously going to get the aircraft checked out by a technician. If he determines that there was infact a small fire/flame during flight, then from the point you have the knowledge of the occurance you would have to notify the NTSB "by the most expeditious means available" . Now, if there is no evidence of a fire, then i'd go with what i posted above.