It's my only plasma so I'm not sure how they die. It happens pretty quickly though, if I spend a bit of time on the pause screen or something, the image will still be visible while I continue playing the game. It's noticeable in RPGs where I have to be in the menu often.Image retention would be a very unusual symptom of a set dying. Any game with a bright or persistent HUD would "burn in" my plasma (quite noticeably), but it would always go away after watching or playing something different for an approximately equal amount of time. I'm honestly astonished you have only recently noticed an issue. Running the screen wipe tool in the menu or looping plasma break in slides can speed up the process if it really bothers you.
It's not really a huge bother, I just assumed that meant it would get worse from this point and eventually die. So I wanted to gather some info on a replacement.