That light indicates that the motherboard is receiving power. And yes it has been there all the time. And this question was also asked here in the forum, so no it was not just you that noticed the LED. 
Optiplex 3010 solid orange light beneath graphics card. PC is completely functional but this light appeared. It may have been there the entire time and i was just not paying attention.    PC: Optiplex 3010 RX 570 I5 2400 8GB RAM  
Optiplex 3010 Solid orange RGB beneath gpu