Where is the smoke coing from? Underneath the car, passenger side, about where the front door is?
On my 560SEL, I got smoke there resulting from a transmission pan gasket leak. Was showing up the way you said, after highway drives and when car was hot.
Look underneath, at the exhaust pipe where it passes the transmission. You would see the pipe stained with burned, leaking transmission fluid, and the rest of the undercarriage around it would be wet with fluid.
Take a look, and please report on other conditions with the car, (ie smoke from exhaust, rough running?)
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)