In a rainstorm you may see water at the top of the foundation running down or leaving a stain.
Superhydrophobic wall siding leaking water.
If it is a cantilevered area as in this case place your hand at the base of the wall and feel for.
Water leaks at soffit or eave overhang penetrations for chimneys that permits water to leak at the roof surface then pass behind the siding if the chimney is not properly flashed.
Vinyl siding installs in pieces so gaps and overlaps.
Ultimately water began to appear on the floor of the kitchen inside.
My photo above of ice below some courses of vinyl siding traces a moisture problem traced to improper installation of a through wall vent for a direct vent heating appliance.
Wood siding leaks damage.
Then using a drywall saw cut along the line you just scored.
Use a utility knife to score a line about 10 inches 25 cm long into your drywall at the location where you see visible signs of a water leak mold discolored drywall etc.