It was meant to be abandoned. That’s what Steve was told in the briefing - he was going to clear an abandoned HYDRA base with Nat and Bucky, so that SHIELD can move in and claim it. That’s how it was meant to go.

Walking down the damp, stone corridors, Steve enters yet another cell block. As he scans the cells, along with some torture rooms in between, he took in the atrocities. Cells with empty shackles and chains, that clinked gently where they hung. A room with a bed, metal straps where the victim's ankles, neck and wrists would go. More cells - completely empty. Another torture room, this time containing a metal chair, with a head clamp attached. He quickly sends Nat and Bucky a status report. As he stops to absorb the atrocity, trying to comprehend that this was done to Bucky, he hears a soft whimpering sound.

What? Where did that come from? His attention is drawn to yet another cell - in the far corner, tucked away from the rest, slightly different in that it was much smaller. In the dim light, a figure pressed themselves away into the corner, as far away from the bars as they could get. It's you.

Another super soldier, subjected to HYDRA's wipes and control, you'd been wiped, tortured via sleep deprivation and electrocution, starved and then literally thrown into this cell and shackled up, left to die. Cuffs were placed around your wrists and ankles, chaining you to the wall, and a tight muzzle was forced over your nose and mouth. You're wearing mission gear quite similar to the one Bucky wore as the Winter Soldier. Are you another soldier? That Zemo mentioned?