House water supply "mystery branch" upstream from shutoff valve
And the answer is.... water line comes in horizontally. BUT, a shocking discovery made by my plumber while sorting this out. Somebody else in the past had the same idea as me and started to remove the downpipe, then abandoned the effort. They made a horizontal cut into the pipe below the T, from behind. It was not easily viewable from the front. I put a feeler gauge into the cut slot and it appears to be DEEPER than the pipe wall thickness. Basically, it's just rust and gunk that has kept the water from exploding out of that slot. I've encountered my fair share of home improvement crime scenes over the years, but this one is right up there... a glob of rust away from disaster.