What could be causing this?

The house was someone else’s crappy Reno project. The master was originally the garage. I think the flooring is straight to concrete but I’m not sure. The pictures are of the baseboard of the exterior wall and this has slowly spread along the entire baseboard over the last three years and I’m just now pissed enough to deal with this project myself. We had mold remediation, roofers, plumbers, electricians, pest services come check this wall during all the other shit we’ve had to deal with and they said it wasn’t caused by leaking pipes, roof damage, infiltrating water damage or termites and no indications of mold. Everyone says to open up the whole wall, but it’s only showing any signs on the baseboard. Is this just from sweating from direct contact to concrete potentially? Any advice appreciated.