The astrophysicist Chris Lintott pointed out that hiding extraterrestrial UFO visits is near impossible on this
episode of Past Present Future. Because the earth rotates, if an observatory (using light or anything else) sees an object moving, it will lose track of it in less than 24 hours. So, as a standard practice they let other observatories on other parts of the earth know, so they can continue the tracking.
The Apollo 11 command module took about 70 hours to get from the moon, the closest extraterrestrial body, to the earth.
The moon is 238,855 miles away, so it had to average about 3,400 mph. (When it hit the atmosphere, it was going 24,000 miles per hour.)
For a UFO to make it from the moon to earth fast enough that it’s only seen by observatories in only one country, I’d think, generously, it would have to make the trip in under three hours. So, superior moon technology would have spacecraft 36 times fast than ours.
If it came from outside the solar system, then it would have to cross 4.67 billion miles in under three hours. That’s about 2.3 times the speed of light.