2 wks ago, brownish stains appeared around the screen of my C710, & keep getting bigger. What is

My acer chromebook C710 is about a yr old. 2 wks ago, I began to see brownish stains forming on the top & sides of my chromebook screen. They have been getting bigger & covering more of the screen each day. The stain continues to come from the top & sides of the screen. I have never had any liquids near my screen so I know that it's nothing like that. My chromebook has always been on my desk, and I've never even taken it outside. It's never been dropped & never had any pressure on the screen. So, I can't imagine what is forming on the inside of my screen that almost looks like an oily stain. And, I am the only one who has ever used my chromebook so I know that no liquid has ever touched the screen. What could be causing this?