The charging port broke on my Moto G3, the battery drained completely and it sat dead until I got the replacement part a month later. After changing the port I plugged it in and...the mysterious LED behind the speaker came on. I switched chargers, it stayed on. After 5 minutes charging, the LED went away, just as the screen, which until now remained dark, turned on showing 0% battery and charging. When a battery is completely drained it can enter a state where it has some difficulty accepting charge, if the battery is still good this remedies itself after a little while. So I'm going to guess that one of the functions of this LED is a low level detection of whether the battery is accepting charge properly. The age of your battery, the condition of your charger, and a variety of other factors can probably set this off.