Our telescopes can only see things thousands of light-years away if those things happen to be emitting zillions of joules of energy per second. Our sun, for instance, emits about 3.846×10^26 joules per second, and if it were outside of our galaxy, we wouldn’t be able to see it at all as an individual star. Moreover, most of the stars that we can see are millions of times the size of the earth.
Basically, our telescopes can see things that are very, very, very big, and very, very, very bright.
As for other things, we can hardly see those at all. Outside of our solar system, we cannot really see anything even as small as a planet. We have to extrapolate the existence of planets by measuring gravitational perturbations and fluctuations in light coming from nearby stars. But we certainly can’t see them, even though many of those planets are many, many times bigger than the earth.
UFO’s, if they existed, are probably much smaller than the earth, and it is unlikely that they would emit very much light at all. Even for the sun to emit as much light as it does, it needs to burn around 500 million tons of hydrogen every second. I doubt any kind of spacecraft would be able to do that.
So in synthesis, there is about a 0% chance that our telescopes would be able to see a UFO, unless that UFO was already so close that it was orbiting around earth.