With the curtains firmly drawn? All day every day? Other than the obvious: a broken window which is unable to be closed (don’t think this is the case due to the below timing).
Since moving in a few months ago, our neighbour’s adult son keeps his upstairs bedroom window wide open no matter the weather. Freezing cold, sideways rain, doesn’t matter. Window stays wide open. And he’s in there most of the day, I can hear him practicing guitar or talking (he has a deep loud voice).
The only thing I can think of are nefarious reasons, eg that he needs the ventilation to grow cannabis (zero proof of this hence the thread looking for other reasons).