Requires iOS 8
For iOS9, try PanicLock9
Also available, PanicLock7
Discrete way of locking your iDevice.
Per-App Hiding in the App Switcher
Per-App Protection in the home screen
You pick a secret gesture, and when your passcode is compromised, you activate panicLock!
Secret gesture can be configured in settings.
Source code for the basic version can be found on Github
Compatible with the latest iOS 8, tested on an iPhone 6 Plus 8.1.2.