This is from an interview with Billy Burke. He mentioned the relationship between Miles and Rachel in this part: [SPOILER ALERT]
WHAT THE DEAL WITH MILES AND RACHEL? | “It’s a-coming,” teases Burke of the characters’ shared backstory. While he hasn’t filmed any flashback scenes with Tim Guinee’s Ben yet, “the relationship with Miles and Rachel is starting to come to light.” And that story might just lead him to some new players like Randall.
So will his journey cross with that of the Department of Defense mystery man? “Eventually, because of his relationship with Rachel,” replies Burke. “Rachel’s heavily embedded into that part of the storyline. We don’t know what the extents and depth are of Miles and Rachel’s relationship yet, but as that starts to get revealed and as we get closer and closer to getting Danny, I’m sure those will intersect at some point.”
Source: TV Line