i've been wondering this for a while, Mass Technology was discovered on Mars before the First Contact War which was started after the Mass Relay in the Sol system was activated (Turians encroached on a human colonized world, we fought back) since it would have been unlikely that the Turians would have been unaware of humanity if they moved an active Mass Relay to the Sol System (can not remember if that even possible without Reaper assistance)
however, before Mass Technology was found on Mars humanity in Mass Effect was probably doing what we are doing now and that's sending probes into space, using high powered telescopes to look around discovering new planets and such
so i am wondering, how did humanity keep missing the Mass Relay in the Sol System