If it only occurred after 30 mins of a static image, I'd switch the TV off for a few mins, and see if it persists. You can get the situation where the LCD cells get electrical offsets temporarily induced when driven in a constant state for periods of time.
You can get image retention/screen burn with LCD, but you have to go some to do it, as it's normall either screen coatings, or the discolouring of one of the LCD filter substrate layers themselves , but normally this is when it's been left on a static image for a long long time..