A leak that stops with one fixture valve is more localized than flow that continues until the house supply is closed.
Watch what changes when a fixture stop, appliance valve, or building master is closed. That response can narrow the active path. Do not open finished walls, tighten corroded fittings, or force a stuck valve simply to make the call sound more certain.
Keep this boundary: A wet spot does not prove a slab leak, roof leak, failed supply line, or drain failure. Recreate the condition only when doing so will not worsen damage.