It's the little suction cup disc that came with it. Haven't had a problem with it falling like my C330 did (though the 750 is quite a bit lighter).
I only have it on the passenger side because my radar detector is on the driver's side above the mirror. I hope to have a ScanGauge2 up there above the mirror as well, soon. I'll be able to look up, know all my info, where I am, etc, and check my rear view...