Hey, that's cool. I've never noticed it before. If you have an lcd display and you look at the screen from an angle they show up really well. I edited your image to make em stand out
Hmm. I don't seem to have letters on my portal though
I don't see it either on my game. Does your screenshot program have a watermark capability or did you actually see it on the screen? I'm just eliminating the possibility it's a watermark from the screenshot or other picture/photo program you used to copy and paste.
It's also possible that it's a background from an old upload.