I've been noticing something peculiar during space stage. Several times I have visited a planet that has the remains of what used to be a creature in civilization stage. There will be marks of a city on the ground w/ a spice crate, yet no city.
It's really interesting. It's as if the previous sentient creature approached some sudden decline that reverted them back a stage, or they met a Great Filter and died.
Sorry some of the pics are rly zoomed, I had to crop them bc the file size was too big.