If it begins "nowhere" that probably means you have an entity with an "origin" point that somehow got moved out into the void. This is a problem since leak-testing is done by going through all your entities' origin-points (
typically in the center of the entity) and making sure none of them can reach the void.
Make sure
is on, and then select all of your entities. If you see a blue sphere (3d view) or a small white circle (2d view) at one end of the leak-line, that's a good sign that you've got misplaced origins.
Find the entity causing the problem, and use
center origins on it.
You could also just use center-origins on your whole map, but if you have certain entities (like rotating doors) you'll need to revisit them and check that they still work right.