system
Given the following post, respond as if you're commenting on Reddit:
human
[Stellaris] Why don't "primitive" civlizations' astronomers notice ships or observation posts?

The tooltip claims passive observation is made "from a safe distance", but it seems unlikely they'd become interstellar civilizations without ever inventing telescopes.
gpt
The thing about telescopes is that they have to be watching at exactly the right time in exactly the right place to see anything even remotely suspicious. The odds of that are usually pretty small so there wont be enough consistent sightings to confirm any theories about extraterrestrials.

Also, starships in Stellaris have detection ranges the width of several star systems. "A safe distance" might be light years, at which point all said telescopes would see was what was in that spot years ago.

Lastly, as hard as the primitive civilization is looking the observing civilization is trying to stay hidden, and it makes a pretty unfair game of hide and seek when the observers have advanced tech including camouflage, stealth and electromagnetic diffusion abilities, and the observed only have the Mk 1 eyeball and some curved lenses.