Argent Tower Secret Bug??

[Gambit]

I am replaying the missions individually. In the Argent Tower, the 5th secret ? is greyed out and I know Ihave all the secrets (miniatures etc). Would this be a bug? How would I get credit for it?

[DaftCanuck]

I have used the powers of Hell to bring back this comment from the dead. The 5th secret in the Argent Tower is a collectable that can be found on the way up the tower in one of the lower sections. It is on a ledge below one of the side support beams.

To note: You can absolutely get locked out of getting it if you progress too far up the tower. If that happens you will have to start the level over again in order to get it.