In the Combine Nexus chapter ((map pht_a15_nexus_a), at the very end, after we fought off a wave of combines and they threw gas bombs into the room, you go back from where you came from and a door opens there, to the left of the door, you will see a table and on it you will see a business suitcase with a cassette that you open and just take the casset with the button E

After you with cassette are bent down together, don’t go forward, but turn back and see a doorway with plywood that blocking the entrance, you can destroy the plywood with all weapon and then go inside the room

After you have destroyed the plywood, what blockaded door , you go inside and just insert the cassette into the player, sit and just enjoy the secret.