The team turned to Parliament's locksmith for help and, with some difficulty, he was able to open the wood panel door, to reveal a tiny, stone-floored chamber, with a bricked-up doorway on the far wall.

Looks like the locksmith picked it maybe?

Edit: from the Guardian: “Liz Hallam Smith, the team’s historical consultant from the University of York, said: “[...] Once a key was made for it, the panelling opened up like a door into this secret entrance.”