Good advice above.
As John Said, Mould smells musty, you can not always see mould.
Probally main cause has been lack of ventilation,
stagnent air and moisture cause mould.
Since learning about mould my advice for a healthy home is;
OPEN WINDOWS. ALOT! (AIR EXCHANGE)
when you shower, when you dry cloths indoors to release moist air. and when you wake up in the morning and the air is full of moisture/co2 from your breath.
Again as John says look for water getting in.
hope that helps