Contrary to popular belief, you needn't have fruit in your house to have fruit flies. All you need is fermenting organic matter that remains wet or moist.
Most likely, that's going to mean drains sinks, showers, etc. In particular, fruit flies tend to breed in slow-moving or seldom-used drains in which a layer of slime has built up. Other good fly breeding spots in your house might include wet areas under dripping pipes, garbage containers, and discarded bottles and cans.
The key to getting rid of them is finding their breeding spot. One way to do this is to temporarily cover your drains with plastic, taping it down firmly. If the flies are breeding in that drain, the adults will accumulate underneath the plastic within a day or two. Once you find the source, you should try cleaning the drain with a stiff brush.
You might also try slowly pouring boiling water along with a cleaning solution down the drain. Even if you don't find the source though, you can take solace in knowing that eventually the flies will disappear on their own. You need only be patient.
