You should only be worried about the fact that there is a fruit fly around. Where there is one there might be more......
If you spot more, make a fruit fly trap.
Fill a wine-bottle with a bit of wine, add some soap
and just let it stand next to your carboys.
Fruitflies will fly in and drink the wine and get killed by the
soap. Or just drown in the wine.
Each year I trap many with this setup.
Here you can find it with pics:
http://wijnmaker.blogspot.com/2009/08/fruitvliegjesval-fruitfly-trap.html
Luc