Default [Q] Pocket sensor/proximity sensor?

So, I installed Tweaky ROM a few days ago, and since then my phone has been waking up if I take it out of my pocket.

This morning, I noticed that when the phone is locked or on the lockscreen, there is a very faint red light I have never noticed before;
When locked

Screen unlocked

Does anyone know what this actually is? My RAZRi had a pocket sensor, but never did this. Is it just a feature of Tweaky?

Thanks
Moto G, Nexus 7 2013 & Samsung Chromebook.