Some clots go quick
As dr. Kuhlman said, some clots can completely recanalize (go away) very quickly, especially if they are calf.
So it is not impossible that your clot might be gone. There is no question though that the location of the clot and the skill of the sonographer makes a huge difference on what is found by an ultrasound scan. Regardless, it is recommended now that you continue anticoagulation.