Right inside the entry, a pool with toxic waste. Jump in and tap the end of the platform. It lowers, and reveals some goodies. Going for the goodies, tap the left wall inside the goodie room; here is the secret. Two independent teleporters, each leading to ammo. You are gonna need it.

Get the red key and get inside the red door. At the back wall, a steel box. Get around it to get the blue key. At each side of the steel box, there is a red torch. Tap one of them. The box will drop and reveal a Nine Inch Nails logo. When you step on that, you have the secret.