Complete Mission 3: Escape
Story related, cannot be missed. You'll obtain this trophy after completing Mission 3: Escape.
Noise and death have followed in your every footstep. A large horde of infected are attracted to it like bees to honey, chasing your scent through the tunnel system.
A signal has been located in the tunnels ahead. You do not have much time left to find Alpha Team and escape the tunnels alive.