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
Best Answer
It appears you are a little bit mistaken - while there were Prothean Ruins (and technology) on Mars, the only Mass Relay in the System was orbiting Pluto; The so-called "Charon Relay" was discovered in 2149, only a year after the Prothean Ruins were discovered on Mars.
Given the level of advancement of humanity before the Prothean Ruins were uncovered, it was not unreasonable to assume that the Charon Relay was missed due to it being misclassified as an icy-moon. After the discovery on Mars, it became clear that both a) there were other sentient beings in the universe, and b) they were capable of constructing structures the size of very small moons, Humanity took a closer look at Pluto / Charon and said, "Huh. Fancy that.".
The Turians were not found on the other end of the Charon Relay, and the first contact war didn't happen until 8 years later in 2158, after Humanity was well on its way with colonization efforts (and had discovered and activated several more relays).