Last night while messing around on Canary Wharf, as a defender on the first objective I was able to knife somebody (twice) through one of the objective doors.
It occurred after one of the doors was blown, allowing access inside. While I was inside waiting for enemy players to come in, I saw someone attempting to plant at the other door (right side, if I’m facing out toward attackers – sorry I don’t know the specific names yet). Their character model peeked a little through the door, so I quick-knifed, hit, quick-knifed again and killed them.
It’s worth noting that my bullets didn’t actually register when their model was peeking through, though it’s entirely possible I just have terrible aim.
EDIT: Thanks, FireWorks!