Ok Headcannon time!

So Steve finally manages to track Bucky down and brings him back to the Tower. Things are understandably tense between everyone and Bucky. 

Meanwhile Darcy and Jane have both been brought into the Tower as well.

Most of the Avengers are out saving the world, leaving only Bucky in the Tower when H.Y.D.R.A decide they want their asset back. Bucky is pinned down and Steve is desperate to get back to him,

When he does, Darcy and her trusty taser is standing there with a stunned Bucky and numerous unconscious H.Y.D.R.A agents.

And that is the story of how Darcy Lewis gained a H.Y.D.R.A profile that only states; WARNING. EXTREMELY DANGEROUS. FLEE ON SIGHT.