Sometimes a shadow appears when

This problem happens to plasma TVs usually due to the Z SUS part, Z SUS board and Z Drive Board, malfunctioning. The Z SUS is responsible in resetting the screen for the next picture to appear. If it malfunctions, the image of the previous picture freezes and appears as shadow. Usually, the last image on the screen when you turned the unit off appears when you turn it back on before the actual picture appears.
The usual fix for this is alignment. That is, adjusting the voltages that powers the boards to an appropriate value that is indicated on the back of the panel. You may need the service of a qualified technician to perform the alignment.
I hope you find this information helpful.