If you were an alien with the technology to cross the vast distances between the stars why in the hell would you need to monitor Earth from within the atmosphere flying your spaceship around? Earth technology exists to read a license plate from orbit. You can somehow build the technology for interstellar travel but can't manage to build telescopes?

For the quarter the cost of Twitter you could build a pretty sweet Earth monitoring station on the surface of the Moon. It makes zero sense that aliens with essentially magic space travel technology couldn't do the same and go completely undetected by people on Earth.

You're assuming that interstellar travel is so difficult that if it can be done, anything else can be done too. It's a fallacy a bit like someone from the 1500s assuming that any civilization that can put a man on the moon must have also cured all disease.

If you can build a spaceship that can effectively travel to another star system you have the exact technology needed to build a fucking telescope when you get there. The very ship you use for the trip likely has a very capable telescope as it would be needed to avoid interstellar debris en route.

The scale and scope of "building a telescope" and "building an interstellar spacecraft" are not just comparable but directly related. I'd also assume such a civilization would have a good handle on diseases, at least have a strong understanding of them.

It turns out that the civilization on Earth that landed on the Moon also had a pretty decent understanding of disease. By the time of the Moon landing many diseases that wracked the world in the 1500s were well under control in the first world. Some were even eliminated.

Telescopes are great but aren't going to tell you much about the exact radar capabilities of a nuclear aircraft carrier, nor what tactics it would use if it did see a craft, nor submarines (one of the incident reports claimed the tictac went underwater). And monitoring Earth is a very general thing, it can even include getting up close with the native wildlife, it isn't restricted as a concept to viewing from a great distance.