Give them an inch, they'll take a mile

All it takes is one.  One open container.  One missing lid.  One spot in the bar that didn't get swept and mopped and wiped down.

Fruit flies are incredibly aggressive.  They can seemingly pop up over night.  And if you give them any excuse, any opportunity, any advantage, they will take it.  Give them an inch, they'll take a mile.

The key word with flies is sanitation.  They love filth.  To control them you must control filth.  If you give them any excuse to live and breed, before you know it they will thrive.

How often do you go through your stock?  When was the last time the storage items were pulled out away from the wall and inspected?  Remember that thing that you were going to "get to later"?  Did you ever get to it?

Flies will be flies.  We have to be so clean that they'll have no choice but to look elsewhere.