Apparently Moto G3s (plus other Moto models) do have a LED behind the speaker grill.
It supposedly cannot be enabled, but it can if the phone has been rooted.
There is this link to a poor audio You Tube video which shows one app (of a few apps) that have been developed to utilize it as a notification tool for when the screen is not working.
There are also reports of the LED turning on after the phone (non rooted) has been upgraded to Android Marshmallow, but apparently it is random, the reason for it is unknown and it cannot be turned off.
I do not know if this will work but if it bothers you, you could try doing a backup of your phone and then performing a hard reset to see if it clears the problem and then restore your phone using the backup you made.
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