Phobos monolith
https://en.m.wikipedia.org/wiki/Phobos_monolith
Good ol Buzz, giving mention to the subject
https://m.youtube.com/watch?v=bDIXvpjnRws
Also, Mars monolith https://en.m.wikipedia.org/wiki/Mars_monolith
Europa is another intriguing spaceball, with the (vague if you must) possibility of a life-supporting ocean.
Edit: add mars monolith