There is a secret area in the steam tunnels that is made out to look like the lair of a Predator. There is a collection of human skulls, 2 Alien skulls, 2 Predator spears, a Predator's helmet, and the flying disc weapon from Predator 2.
When Caleb is on the rooftop above the museum and looks down through the glass, you can see images of fish carved in the walls just under the glass panes. These are images of the killer fish from the first Blood.
The character "Gabriella" was previously a man, "Gabriel". During a conversation with Caleb she says "It's a long story".