mobile phone causes mbp to sleep

MacBook Pro (Retina, 15-inch, Mid 2014)

10.10.5

There is a sensor under the left speaker grille. When my mobile or tablet is placed near or on the top of the notebook, it causes the mac to go to sleep. I haven't been able to tell if it is just the display sleeping. This happens even with Caffiene set to prevent sleep, so it seems to be hardware thinking the lid is closed.

What is this sensor? I assume without a physical latch it may respond to something behind the screen bezel. Touching the tablet or mobile to the screen doesn't cause the same results. If it is some ambient light sensor, for the keyboard backlight for example, how can I make it stop since I do not use the keyboard backlight nor do I use the sensor for screen brightness.