How about If they are in disguise? In fact, the laws of physics tell us, no matter what kind of space aliens with advance technology, can’t hide it generates heat. So we just make infrared telescopes, we can easily find this heat. To make a city the size of a ship in part to the speed of light flying alien spacecraft slowdown will release a great deal of heat, we need to find this is the part of the heat. Astronomers are still in the search for extraterrestrial civilization by looking for Dyson sphere waste heat. Dyson sphere is a kind of can put the whole stars, even all the stars in the galaxy, in order to get all stars energy device. But so far, nothing find.
However, if the aliens want to bypass the alert is close to us, just use one of the most ancient tactics – they can fly to us under the cover of the sun. A considerable part of the earth the sky area is dominated by the hot plasma ball. Astronomers sometimes only in an asteroid have found that they flew over the earth. That is because they like intelligent aliens, is also appear to come from the direction of the sun. To find these asteroids, space-based survey equipment is essential, they can use different perspective to look at the sky. In fact, we do have such a plan.
So even if our technology is so original, still have a chance to before the arrival of the alien invaders ship found them. It is best to have automated observatories and space-based survey equipment. And, of course, if hostile aliens are close, we can also take some special defense way — let the fog haze hangs over the city, for example — that aliens can’t see anything.