As many movie watchers keep wondering about Hawkeye's whereabouts in Marvel's phase 2 and filmmakers keep confirming, those will be revealed in the upcoming Avengers: Age Of Ultron, fans of the sharp eyed archer get at least a very subtle easteregg in Captain America: The Winter Soldier.
Directors Anthony und Joe Russo in a long podcast with Empire addressed that the arrow necklace
that Scarlett Johansson's Black Widow wears throughout nearly all of Captain America: The Winter Soldier, is indeed a reference to Jeremy Renner's Hawkeye and exists for good reason.
“Honestly it was a choice that Scarlett made as a character, and… her relationship with Hawkeye will become very clear in Avengers 2.”
So not only this carefully chosen costume detail shows that Black Widow's and Hawkeye's deep connection, as explored in Marvel's The Avengers, is still very prominent. But also it's promised that we'll get to see more of it soon.
Scarlett Johansson herself also commented on the subject in a video interview on rotten tomatoes.
"I think that may have been my little nod to Mr. Jeremy Renner’s Hawkeye. I guess you could interpret it in different ways but we thought it would be nice to have something that was a little bit personal for the Widow. You kind of forget, because she’s such a badass, you kind of forget that she’s still a woman and that she has her interpersonal relationships in life outside of work.”
While of course all this was to be expected, especially since fans already spotted the necklace on paparazzi set photos a year ago, it's nice to have official confirmation that continuity concerning two two master assassins and their rich past is not forgotten in the Marvel Cinematic Universe.