You need to do is find the source of the water (I assume) whether it be coming from a pipe, drain, or fixture, or a roof leak.
If the leak is constant, it’s likely a plumbing issue. If it shows up after a good rainfall, then I would be checking the roof.
If this wet spot appears to be coming from the middle area of the wall, I would open it up right where the wet spot starts. Probably a pipe leak in wall that is spraying.
Otherwise, keep working your way up and realize water goes downhill, which may not always be a straight line.