most of these small flies don't live very long so the chances are excellent that somewhere in/nearby your flat there is something which has been/is harbouring eggs. The eggs could have been laid last year. It varies with species but almost any food residue (or poo), especially rotting fruit, will do for flies to lay their eggs in, and they are attracted to the smell of anything similar, including ripening fruit in a fruit bowl. Recent slightly warmer temperatures may have prompted eggs to hatch out, or adult flies to be attracted to anything in your flat that smells nice to them. If the infestation has been going on for more than a certain length of time, you may be one or more generations into the infestation.
So you need to be absolutely sure there is nothing which might be harbouring eggs and equally well nothing which will attract the adult flies, else it'll be ongoing.
A kind of trap for the adult flies is to leave a piece of stinky decomposing fruit somewhere, but to change it every few days. The flies will spend their time laying eggs in that, and you will dispose of it soon enough that those eggs will never have a chance to hatch. If you can bear the smell, microwaving the fruit will kill any eggs that have been laid.
cheers
