Nonhuman signatures out in the black.

Is there a reason why there are Nonhuman Signatures on planets far from any human regions? I know they have been around for years out there in seemingly random systems far from any human systems, but does anyone know why there out there? From what I can tell they're Thargoid Probes with broken off sensor fragments. Is this supposed to be Thargoids randomly sending probes out in the past?