if it is a rather old building, it probably has plenty of drafts round the windows, up the chimney and under the floor, so the second most usual cause (lack of ventilation) probably doesn't apply, and if it's been empty, the first most usual cause (wet washing draped around) shouldn't either; except that as water vapour is lighter than air, if you have downstairs neighbours who like damp, and put water over the radiators soaked into wet clothes, then you will be getting the "benefit" of their humidity. Is there any sign of that? Condensation mould occurs mostly on external walls, usually high up or behind large furniture, and not near sources of ventilation.
If not, then in an old house I would be looking for signs of water ingress; through a leaky roof, around windows, through cracks in walls, or from plumbing especially in bathroom or kitchen, but also from radiators. If you air it well so the smell is gone, then sniff around the floors and walls, you might be able to detect where it's coming from.
Bleach kills mould temporarily, but if the surface remains damp, it will keep coming back.
The bathroom ought to have an extractor fan.
Summer will soon be here so you can keep the windows open more.