A slight spark is normal. Flow of current with key off is normal in our cars as several systems like BnMe, Immobilizer, Instrument panel, CAN, radio player etc work all the time in Fiats. If a relay is malfunctioning you would face starting trouble after parking for 5-6 hrs. You would not have noticed the spark earlier because of spray done on battery terminals and clamp by TASS during service which covers the area which is exposed to nature. Usually spark happens when the outside of wire clamp touch the battery terminal loosely.