Before we get down to business, here are a few essential things to keep in mind as you search:
A high-security zone with keycard-restricted rooms.
When the alien ambush begins in the courtyard, look for a cracked garage door. Throw a grenade at it to blow it open and reveal a safe zone with turrets and health.
Finding these secrets is the key to surviving the later stages of the game. They contain massive cash drops, high-tier armor, powerful implants, and advanced weaponry that you otherwise couldn't afford or access early on.
Watch your mini-map for "black space" that looks like it should be accessible.
While not a "level," there is a famous farming strategy often referred to as a secret spot by the community.
Blast the wall behind the first computer terminal you encounter.
Occasionally, a secret area will contain a permanent stat boost. These are the most valuable pickups in the game.
Inside the clean-room laboratory, search for a wall section displaying a faint red flashing light. Destroy this wall to find a secret stat-boosting implant. Mission 5: The Weaponry
Use this quick-reference checklist to ensure you've found every secret:
Some secrets are hidden in deep shadows at the very edge of the map where you can't see the floor. Steam Community