As we dig down, the crushed rock is disappearing from the earth along the foundation wall. We're about thirty inches down now and digging through silty sand with occasional rocks and construction debris. It does not appear this was ever a proper foundation drain, or if it ever was, it has thoroughly silted up. There's still the possibility of a drain way down the bottom at the footing, and the pipe inlets argue for it existing somewhere, in theory ... but if there is one, it can't be doing much good. I doubt there's much drainage through this stuff; there's too much silt and clay in it. The foundation wall has been sprayed with some kind of asphalt-based or tar-based "waterproof" coating, but it's pretty thin and patchy and has holes in it.
The proper fix would be to bring in a backhoe, expose the entire foundation wall, and redo both the drain and the waterproof coating. However, we can't really justify that kind of cost right now, and it would be of limited benefit anyway without digging out the entire basement floor, laying down a water barrier, and pouring a real concrete floor.
For now, the next best thing is to lay a proper French drain two to three feet down, line the ground side of the trench with porous cement blocks to keep most of the earth out, and backfill the entire trench with clean crushed rock.