Why is my iPhone 4 unlocking itself?

About two months ago I bough a new iPhone 4. Recently, it has been unlocking itself (I do not have a passcode) The phone can be placed in a bag, locked with the screed dark, and then pulled back out a while later for the screen to be unlocked and the apps showing (I do not have my phone on a lock setting; it is unlocked until you manually do it yourself.) it has also been unlocked during the night; I've woken up, knowing I locked it the night before, to flip it over in the morning to see the home page or a particular app open.
Any reason why it could be doing this?

This is really strange.

First see what happens after setting a passcode.

If it persists then reset the phone in iTunes.

  • Why is my iPhone screen slowly dining itself? Ok so the screen gradually but by bit got dimmer and dimmer and then stopped so now the screen is bright but not as bright as usual, why is this?
  • Why is my iPhone 4 always shutting itself off when not on charge? Today I got a new iPhone 4 and for the first hour or so it was working fine. It kept turning itself off whenever it wasn't on the charger but it was already fully charged. I would turn it on then it would just turn straight off. I need help.
  • Why does my iphone screen keep controlling itself? So my iphone is in horrible shape, in terms of a broken screen. It's having a hard time responding to touch and I can't type. Also, it keeps opening apps on its own. Someone told me that the screen is pressing up against the digitizer and is acting as if I was touching the screen due to the cracks. If so, can I just replace the broken screen or do I have to buy another phone? I want to wait on the iphone 6 to use my upgrade.
  • Why does my iphone 5s randomly restart by itself? My iphone 5s has been randomly restaring for about 2 weeks now on its own. I tried resetting my settings to see if it would fix the problem, but it didn't. The phone is new. What can be causing this? I'm running iOS 8.3 btw