Can leaving windows open cause mold?

So I just noticed that some of our windows have some mold around them (not bad though). But they are mainly the windows that my fiancé will leave open and forget to shut and I told him it’s probably from them constantly being left open when it’s raining or snowing but he doesn’t think so. His man cave (one of our spare rooms) windows have the mold worst than any of the other windows and he constantly leaves the man cave windows open (he almost never shuts them). We will be cleaning them with bleach tomorrow but I’m wondering if anyone knows if the windows being left open is what could’ve caused the mold. Also our windows are newer only a year and a half to 2 years old.