Events in Stellaris will not be limited to the Anomalies you find in space. Another event category that you will encounter as you play the game are colony events. Fairly self-explanatory, colony events are events that can trigger on your colonies. Our goal with these events is to provide a bit of identity to the planets you colonize, and set them further apart.
Some events will have situations that you can respond to in a number of ways, and in many cases you will have to deal with the fallout (positive or negative) of your choices in follow-up events that can fire many years later.

ISHIKAWA
I’m not sure I see how this pertains to the situ-
BORLAND
The orbital surveys our Science Ships perform only reveal the tip of the iceberg. Once your colony starts growing, and you have thousands of colonists running around in the wild building settlements, you will sometimes run into things that weren’t in the survey reports. Look at what happened on Las Veredas, for example.
SWEENEY
I’m not quite up to speed on events from that sector. What exactly happened there?
HAGNER, L. TIMOTHY (Senior Investigator, BRSF)
I believe they found an old drone on the surface.
BORLAND
Not just one - there was a whole fleet. The drones evidently thought our colony belonged to whoever had originally built them, as they began to sporadically assist our colonists by gathering minerals and terraforming uninhabitable sections of the surface.

SWEENEY
But it sounds like that had a positive effect on the development of the Las Veredas colony. What happened on New Albion is quite the opposite.
BORLAND
Of course it is, you pompous fool! We’re dealing with the unknown here! Our colonists will sometimes find that the planet they settled is even more valuable than the initial survey indicated, like when prospectors on Acrisia unearthed an intact battleship.
HAGNER
Really now, Mr. Borland, you need to -
BORLAND
Other times, the outcome can be disastrous, like when the colonists of Xianyang activated that ancient abandoned terraforming equipment, only to discover that the aliens who built it breathed ammonia!
GIDDINGS
Sit down, Mr. Borland! Sit down, or I will have security restrain you -
BORLAND
So what do I think happened on New Albion? I haven’t the slightest idea! Maybe the colonists were enslaved and brought underground by some kind of subterranean civilization. Maybe they were all consumed by a type of predator that hadn’t been encountered before! All I know is that while you’re sitting here safe and sound on your fat asses in low Earth orbit, there are… no, get your hands off me! Let me finish! There are colonists out there who -
GIDDINGS
Remove Mr. Borland from the room! The rest of us will reconvene in ten minutes.
BORLAND
<unintelligible shouting>

Next week Game Director Henrik "Doomdark" Fåhraeus will tell to you about War, Peace, Influence and Claims!

https://forum.paradoxplaza.com/forum/index.php?threads/stellaris-dev-diary-16-colony-events.901906/