Ok, quick overview of the situation: An alien planet, reaches basically the same stage we are at now in regards to fossil fuel reserves and environmental loss, a major war breaks out, and after both sides suffer basically equally, they decide to merge forces and revert their damage on the planet. buildings are ripped down, power-plants dismantled, holes filled, forests replanted (relatively) and any damage they can reasonably reduce is acted upon. Most of the population then leaves the planet, now with plans of colonizing a new planet, with a sustainable economy. The odd 2-3 billion or so remaining population then create a great city, far underground, to let the planet heal and periodically (as in every few years) monitor its progress. Thousands of years pass, forests and other environments are basically completely healthy, and humans orphaned of their planet colonize this alien one.
After that wall of exposition, here's the problem. In this story, the humans only realize the planet is inhabited about three generations into colonization, and this is only because the natives noticed them. How, after mining and building their own civilization, do these humans not notice the scars created from mining and past cities?
Things to note:
- The humans came on a makeshift craft, a last minute escape if you will, the economy quickly crashed within the first two generations due to competition with agriculture and alien weeds/pests
- Pests are beginning to be controlled at present, as well as development of alien crops, so the economy is slowly rebuilding, but the civ is basically scattered towns and villages with one main capital city. This means the civ is actually quite small, and most of the planet is unknown exploration-wise.
- Humans don't really have resources to expend on expansion or exploration (someone already mined most of it) and instead focus on military, agriculture and infrastructure. This means any satellites are cheap and specifically for communication, not thermal imaging or the like.
- The mothership isn't an option, it is still in orbit and has been completely stripped of parts and energy reserves.
- The city being hidden isn't the focus, this is specifically about the mining and infrastructure scars, however feel free to add such information to your answer, as long as it's a side note. I might make a new question for that topic in the future.