You are heating up water and see bubbles form on the bottom of the pan. Is the water considered to be boiling?
Contrary to popular belief, you don’t have to 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.