Pocketed iPhone 5 auto-locks my screen temporarily?

Since the ios8 update, I've noticed that whenever I put my phone in my pocket unlocked, my phone makes a noise, and locks the screen.

As soon as I pull it out again, the screen comes back and is unlocked again, almost like it's screensaving and helping me not pocket-dial or anything. Great.

My problem is: it always makes the loud noise whether my phone is on vibrate or completely silent - which is very annoying at work.

I can't find this feature even listed anywhere online, never mind a solution to removing it (if you can) so would love to know if anyone knows how to shut it up.

Thanks