Depends on what you mean by turn off.
The SD card slot has a kind of button inside. It's what makes the card click in and click out. Whenever that button's pressed it tells the system the card was changed. So the system remounts it.
It might not notice this button action when in standby. (screensaver)
It shouldn't matter when the device is actually turned off entirely. (blank screen followed up by bootup sequence when you turn it back on)