Plumber or Basement waterproofing company to start. Could be a leaky pipe, could be inadequate yard drainage. Has it rained a lot recently in your area? I've heard of landscaping, french drains, and gutter issues causing water in basements before, so it isn't necessarily a leaky pipe.
The ceiling is above grade. It's set up like a daylight rancher, except there's a third level. It's a long way for a leak to travel without showing itself. The wet wall is ten feet away.
I suppose a plumber could introduce some dye into the main supply and at least prove whether it's a leak from pipes or for some other reason.