Many assume UFO sightings are commonplace. They are not. Ask MeteorWayne, who has never observed one despite all that scope time. Perhaps he lives in the wrong neck of the woods. Back in the '50's, astronomers had a higher percentage of UFO sightings than the general public. I don't know if that holds true today, but it does make sense that those who spend long hours watching the sky would be more likely to see unexplained objects. In fact, according to Edward Ruppelt in The Report on Unidentified Flying Objects, astronomers did see unexplained objects, but some were reluctant to talk about them lest their reputations be damaged.
Telescopes typically are focused on specific targets in space, and it would be extremely unlikely to spot something like a spaceship. Has anyone ever accidentally spotted a man made space probe in a telescope? Not to my knowledge. And how fast do they travel? Typically less than 50,000 mph. Try spotting an object traveling near the speed of light. Or even at superluminal velocities. By accident. Impossible.
UFOs are actually a rare occurrence. The odds of seeing one on a space mission is minuscule. We don't know if the military can detect them entering the atmosphere, and if they can, they are not saying. I, too, would expect that something a mile across would be easy to pick up, but if the government isn't talking, we'll never know if they have.