The next day, same story. The flies were now appearing throughout the day. They all looked the same. Where the hell were they coming from? The windows were still closed. I kept vacuuming.
Eventually things were starting to get very irritating. I hunted for an entry point for days without finding anything and the flies just kept on appearing. I was pretty good at vacuuming flies by this point.
By day six I spotted a group of them hanging out near my Kentia palm. Aha! A fly had laid eggs in the soil of the plant. I had no idea that it was a suitable food source for larvae. Needless to say I quickly filled it with gravel and I guess that's the story of how I became a qualified Fly Detective.