Actually some anticoagulant poisons stay in the body for up to 2-4 weeks. And the way they work, they don't do permanent damage. As long as they are in the system in an active form they interfere with blood clotting, but once they are out of the system they are no longer a danger (unless the dog has bled enough in the meantime to be life threatening). That's why if you know what happened in time, this type of poisoning is actually quite treatable (with anticoagulant rat/mouse poisons only, there are others that have no treatment).
Also, a necropsy may have shown if there was another cause for the bleeding. IME, while not impossible, vomiting blood is pretty uncommon with this type of poisoning. Usually dogs bleed into their lungs or abdomen, or have extensive bruising on the skin.