You are on a mission trip, without the S.H.I.E.L.D employees realizing, H.Y.D.RA has invaded the shield, everyone is unaware including the Avengers team, Hydra has not carried out the destruction mission, when you come back from the mission the shield employees greet you warmly you are a thorough type of person you notice there are so many new faces

You realize that they are not real employees or new recruits, you can recognize Hydra because you have seen some of them while on other missions