hi just had a similiar issue my boy races a lrp 5900 lipo ran last week removed from car after race (so only about 2/3 charged came to charge this week 1 dead cell no puffing.cracks or visual signs of anything amiss (it could be a internal wiring fault possibly )all i know is it cried enough.
yet i have some lipos that i fly with that are probably 5 years old now dont ave a balance lead (not fitted from new from a time when balancing was un heard of ) and they still perform well some punch has gone admiddetly, and have used and abused them mercillessly (safely i may add)
i come to the conclusion you get good uns and bad uns balancing makes little difference if they are starting to go out of balance thats because they are giving up the will to live
just my personal viewpoint and experience no professional qualifications to back up my findings
dave