Be very sure that you haven't overlooked a carrot or something in some dark corner of your cupboards. Fruit flies especially love sweet root vegetables like carrots. The females are the ones you have to worry about. They will lay their eggs in healthy, ripe fruit and veg, which then rots quicky and turn into the kind of nasty stuff that the larvae like by the time they hatch . Throw out anything that looks like it might have been compromised.
The fruit flies go through their entire life-cycle in two weeks. The adults only live about 5 days. so if you can prevent them breeding , they will quickly die off. Best to keep fruit and veg in the fridge, where the fruit lfies can't get at it, insofar as that's possible.
Failing that, it's said that that rinsing the fruit veg in a solution of bicarb will make it unappealing to the fruit flies, though I don't actually know if that works.
Last timeI had fruit flies, they'd come with a bag of sweet potatoes from the supermarket, one of which had been infected. That's what's most likely to have happened to you, IMO. Fruit flies seem to be becoming an increasing problem on the farms (especially in Oz, which has been overrun by European species of fruit fly ) but it's worth giving the drains a good clean, just in case.
Finally, you don't use e-liquid at all do you? Nicotine is absolutely deadly to fruit flies. I've taken to using a bit vineger or something as bait, and adding a bit of high nicortine e-juice to it. No need to trap them. They die within a few seconds of absorbing the nicotine.
I also think that's prolly kinder than trapping them , since it's so much faster.
