When you reach the 'energy bridge' and have pushed the two buttons, the rightmost prisoner cage has a crack. Shoot it, the bars blow out and a secret room opens.

When you finished the level, just to the right of the entry door one of the prisoners have paintet 'kill me' on the glass. Shoot the window and enter. When you crawl trough a corridor, a strange message pops up: "A.H.D.S.S.I.B.H. - bjjc"
Bozhinsky sent me the solution: "A Hall Displays Secret Snapshots In Boss's House - Brandon James & John Carmack"
...So, i assume it's a reference to '#1' in "Final Showdown".