Secret Places (3)
Just outside of the beginning tunnel is a crack in the wall. Blow it up to find a large secret place.
Next to the room with the nightvision message "Under the Knife", duck and press on the leftmost cupboard... under the knife.
The last secret is behind the framed picture near the red access card. It's activated by a touchplate first, so run around the top of the stairs to open it once. Then you can open it at will by pressing on it.
Secret Messages
"Under the knife". Nearby the hooker, inside the building on a dark wall (nightvision goggles required).
"You're not supposed to be here... either - Levelord". Outside of the nukebutton area, floating above a group of ventilation fans (requires DNCLIP).