There is a few causes of this:
* Even though LCD's don't have the same issue of burn-in as older CRT's you still can get it! If you have a desktop image which is static with large dark & light sections you can get this effect.
* If someone was servicing your system and pressed on the back of the LCD they could disrupt the liquid causing a dark splotch or line if the object was sharp.
* Some displays glass plates did not get properly washed during the manufacturing process. This was a big problem for Apple with the 2011 21.5 & 27" iMac screens, some where also off color.
In this case I suspect it was your background image that burn't in.