If not, looks like someone has swapped your "head unit with a CD changer" to a "nav head unit", probably from a scrappy?
I know the thread is "stupid question" , but I won't ask if you have a massive square hole in the top of your dash :rofl:
The single cd units (business unit) sometimes come up on here, or ebay, or you can go aftermarket with a fascia plate
First one on a google search
http://www.ebay.co.uk/itm/Connects2-CT24BM06-BMW-Z4-03-CD-Car-Stereo-Single-Din-Fascia-Black-Facia-Panel-/261172436537
then you can go MP3, SD card, HDD, whatever.