Your phone might be saving you some battery; there is a sensor to detect when your face is close to the phone, turning off the screen and disabling touch. Not sure if it actually works when it's in your pocket though. There's a serious "light in the refrigerator" problem, where trying to measure it changes the outcome
It's really easy to see if that sensor is active. It's not fridge light at all, it triggers at a distance and you could test against your shirt to see if the light shines through.
I've never seen it be active outside of a phone call.
