Nas, I'm not liable for any consequence you may face that could arise from your discreet video-taping, but this is how you do it while the screen is locked:
Make sure your iPhone is locked. Press the Shutter button, slide the bottom right camera icon halfway up and don’t let go. Keeping your finger on the camera slider mode still halfway up, slide the camera mode to video.
Double press the Home button three times.
Keep your finger still halfway on the slider until the iPhone turns off its screen. Remember: your iPhone is already recording a video.
If you want to end the recording, simply unlock the iPhone as usual.