The phrase "faint outline" refers to a very light and blurry version of something, like a shadow or an image that is not clear or easily visible. Full definition
I saw a very faint outline of the navigation keys; it's hardly visible. (digitaltrends.com)
You can see faint outlines of the image that was there, as well as being able to see a slight ghost of the text at the very bottom that instructs you slide the switch to wake. (manga.jadedragononline.com)
On Google Maps, the triangle - shaped, 53 - acre lot squeezed between Los Angeles» highways 110 and 405 is still a big dirt patch with faint outlines of the road course etched into the earth. (automobilemag.com)