The plastic sheet with funny markings is primarily clear to carry the LED light across the underside of the keyboard, the funny markings catch the light so that the keys are illuminated evenly (you’ll notice that the dots are larger at the periphery of the keyboard where there is less available light).

The unknown sensor is the real motivation for my comment. The plastic backlight diffuser is pretty cool, but the sensor that you’ve photographed is the clamshell contact - way cooler in my opinion. This is what causes your MBP to sleep when the lid gets closed. Keen observers will notice that the machine will actually sleep BEFORE the latch engages because this sensor is magnetic. You can even fool your machine into sleep with a weak magnet (like a fridge magnet) over this area. There shouldn’t be any danger in trying as the HDD is on the left of the machine, but caution is always advised.

Thanks for the great photos!