Neither of the existing answers really deals with how some guy from the 31st century having found Captain America's shield results in it being in the Guardians' ship in the 21st century.
During the Guardians of the Galaxy run from 2008-2010, Vance Astro (a.k.a. Major Victory) from the 31st century is found, encased in ice, in space by the Guardians (in issue #1). he joins the team, and remains a part of it until the team dissolves following The Thanos Imperative (which concluded both the Guardians' series, and the Nova series that was running at the time).
I glanced through the series, and I don't see a clear explanation of how Vance wound up in "his past" (well, actually, he'd be from a different timeline - the future Vance Astro had met himself in the present, and altered the timeline so that the present Marvel universe (aka Earth-616) couldn't be the universe that the future Astro came from).
From what I did see, it's likely Astro probably came back from the future due to the impact of the Rift created during the "War of Kings" storyline - the one that lead to the Cancerverse, which was dealt with in the afore-mentioned The Thanos Imperative mini-series.
As near as I can tell, the future Vance Astro was still alive and active (and still had Captain America's shield) when that mini-series ended. To the best of my knowledge, we haven't seen him since then (in our time, at least).
So, it's possible that the shield in the Guardians' ship is the one Vance Astro brought back from his future. Since Vance was a member of the present-day Guardians along with Rocket, Groot, Gamora, Drax, and Starlord, it's possible one of them wound up with the shield, if something happened to the future Astro between the end of the 2008-2010 series, and the start of the 2013 series.