The budget to look for ELEs is still rather tiny. Which means the vast majority of telescope-time is ground-based, which means we can basically only look at the sky that's the half that is away from the sun. So anything that takes the trip through the inner solar system in under 1 year can easily sneak past our telescope efforts. We won't even be looking in its direction. Its the planetary version of a scary movie where the baddie walks up behind the character.

And even after the 1.2 billion dollar space based telescope that NASA is working on making, we will still be blind behind the sun and a few other narrow but many billions of directions on the sky (difficult to tell if a tiny light right next to a bright star or nebula is a rock, without massive computing power and individual light-spectrum analysis of every single light source on the night sky).

We likely won't have really good coverage before we get two or three or four such 1.2 billion dollar space telescopes going. Then they can take pictures with all of them directed at the same place at the same time and we'd only need to analyse pictures that differ between each telescope because everything outside our solar system has very low parallax.

Until then we're just holding our eyes closed on Earth going 67 000 miles per hour around the sun, hoping nothing hits us or vice versa. And the planet is a big gravity-well that sucks everything towards it.