Water leaking into house through sliding glass door, could missing flashing be the cause?
Since you have poured water onto the tracks and no water leaked inside; check a few more things. I don't see a gutter above the door. Use a step ladder and run some water from a hose onto the roof, downward toward the door. The water could be lipping over at the edge, into your soffit, across the top of the door and then down the sides onto the floor. Remember, water takes the path of least resistance, so it could be leaking into the soffit from somewhere other than at the doorway. Also check both sides of the sliding section of the door for any cracks that a side wind could blow water through. I would also suggest caulking that large crack across the bottom of the door...