I just bougt a 1987 420SEL. The car has 99,000 miles on it and is in pretty good shape. I drove the car for about a week with no problems untill yesterday. The second day I had the car on the road I added some injector cleaner to a full tank of gas.
A day or two later I drove down to the beach, about an hour and a half away. I came back the same night and the car was great exept the coolant level light came on and off a few times. I checked the levels and everything looked OK.
The next day, after my 1/2 hour commute home I stoppped for some gas. About 2 blocks from the gas station I noticed some white smoke around the car. I got a little nervouse and drained a little coolant and added some fresh antifreez and water. I smoked all the way home.
The next day.......I checked the coolant. With all the levels checking out OK I drove to work. The car wasn't smoking? I got all the way to work with no problems! On the ride home after getting off the highway I stopped in a lot and it was smoking just a bit. Within 10 min after noticing the small amount of smoke it had turned into a huge amount of smoke? ALL the fluid levels are fine, the car has plenty of pickup and the smoke doesn't smell? WHAT IS GOING ON????????!!!!!!!!

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?)

Good luck!

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1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)

Crazy thought - could it be the water output from your air conditioning hitting the exhaust pipe?