This would suggest that some, if not all, are actually taken to another location or into hyperdrive while the examinations are occurring. This seems entirely plausible since the aliens would not necessarily wish to remain on the ground during a medical examination requiring an hour or two of earth time. Too much danger of being seen by others. Besides, remaining in Earth's polluted atmosphere may be unhealthy for aliens accustomed to breathing purified gases. The UFOs and TLOs appear and vanish instantly, that they perform dramatic manuevers while in the atmosphere and visible to humans is well documented.
Thought can travel to Mars instantly.
Thought can travel to the most remote corner of the galaxy instantly. There is no physical motion, no brute force required to send you from one point to another. No humming or buzzing of equipment or engines. You are here, then you are there. You appear and disappear as quickly as you can think it.