Beetle in the Anthill by Boris Strugatsky

In the far future humanity explore distant planets while encountering different life forms on some of them. The scouts of the explorers are called Progressors and are highly trained in all forms of self-defense. One of these Progressors, Lev Abalkin secretly came back to Earth and for some reason it causes a lot of panic among high-ranking security people. Maxim Kammerer works for a security agency. He is assigned to a task of finding Lev Abalkin. Actually, he has two tasks: to find the rogue Progressor and to figure out what sort of danger he poses on our planet. As he investigates, he realizes the extraordinary measures were taken by somebody to keep Lev Abalkin as far from Earth as possible. It all might sound like a sci-fi thriller, but it is actually more mystery book set in the future with the main question: who is Lev Abalkin?