What does it mean if the doctors say they don't know where a pulmonary embolism came from?

What it says: A blod clot in the lung normally did not form there, it got there after traveling from its point of origin. 90% of these clots start in the veins of the legs, the rest come from the arms, or the pelvis. Alternatively, it might be a clumsy way of saying that they don't know why it happened.

Origin unknown: The origin of pulmonary embolism is commonly from the deep veins in the legs, but they can occur from other locations as well. Unless you see a deep vein blockage on a venous ultrasound, the origin of the pulmonary embolism may never be known.