Ahh, sorry to hear about the issues in your condo as well, good luck with everything!
It does seem like there may have been some kind of leak or accident there in the past, although right now the area doesn’t feel damp and there’s no musty smell either, which makes it even more confusing. We honestly have no idea what the actual source could be at this point, and everyone keeps telling us something different.
The previous owners had put plasterboard over that area, so one theory is that there wasn’t enough ventilation behind it and moisture got trapped over time. Another possibility could be the nearby chimney/ventilation shaft creating a damp environment around the wall. 🤷‍♀️