L light is continuously on

I am wondering why the L light is staying on while my program is running. I am not using pin 13 or an LED. I have the updated driver software and am using Windows 7 on a PC. This was not happening while running the same code for 3 weeks, and has just started. Thank you for any help.

It's simple enough to turn it off. Put the following code in setup().

  pinMode(13, OUTPUT);  
  digitalWrite (13, LOW);

The LED is driven by an op amp/comparator. The Opamp input is connected to D13.
If D13 is not used, the pin may float to a high enough level for the op amp to see it as high and thus cause the L LED to turn on.

Setting D13 as low output as dannable shows will keep the LED off.