Northwest of the White House in Freeroam there is an intersection at G. St. NW / 19th St. NW that you will hear "Access Denied Keycard Required". I'm thinking if said keycard was found the street floor metal doors would open so you can descend.
Has anyone found a keycard that this may solve this mystery?
I took a look at that intersection and did not get any message. That corner is also very close to the Bank Headquarters building, which is one of the missions that requires a keycard (two if you count the loot room). My guess is that the Bank Headquarters mission was left before it was finished, and the street corner is directly above one of the locations that require a keycard in the mission. So its possible as you traveled over the vault, it triggered that message.