Or someone later wouldn't know about us.... outside say an abandoned car floating through space...
That would be an interesting story. We travel into space to find things out there only to find things that seem to have come from... home.
The cars we left on the moon will still be there. The satellites and trash in orbit would still be there. The satellites orbiting Mars would still be there.
We can see clear evidence that life has altered the climate in the past [1]. Anyone looking like we are would notice our mark on the environment.
Maybe just the geo-sync ones, but I imagine anything closer will decay in “relatively short order”.
https://space.stackexchange.com/questions/5583/can-an-artifi...
Low-Earth orbits will decay over periods ranging from years to millennia, based on residual air resistance.
Those effects are minimal in geosynchronous orbit, though there's still some residual atmospheric drag which might be significant over millions to billions of years. The period under concern is roughly 5 billion years, at which point the Sun will have ballooned to a red giant, enveloping the Earth. (Increased solar radiance, as well as various cycles and changes on Earth will likely have long made it uninhabitable by life within the next 800 million years or so.)
Other effects include solar wind, possible interactions with the Earth's magnetic field, precessional effects caused by the Earth's axial tilt and irregular geometry (to use that word in its literal sense), as well as gravitational perturbation from the Moon and possibly other bodies within the solar system.
There's a slight possibility of impacts, though those would be highly unlikely -- space is big, as Doug said, and satellites tiny. Even the Earth only gets wallopped every few tens of millions of years. Though any such impacts might create a sufficient debries field within the orbital zone to affect other satellites. The Geostationary band itself is quite narrow, and preferential placement is above the equator, which would focus placement for potential interaction.
It's an interesting way of thinking of the age of the earth
and geological changes that there could have been a
civilization and we wouldn't know
I guess the most pressing evidence is that nobody used the fossil fuels before us (at least in large quantities), and a lot of it is from the time that plants first colonized the dry soil.
If somebody got it before us, they must have got very little of it.