What is causing my Lexus to smoke when I start the engine?

I have been a problem with smoke coming from the exhaust for approximately 5 years now. Whenever the car sits idle for about an hour and the engine is started, there is always a lot of greyish smoke coming from the exhaust. The car dealer looked at the problem, acknowledged there was a lot of smoke during their check ups, but could not find anything wrong with the engine nor the cause of the problem. This is very embarrasing to say the least and I am concerned there may be a serious problem that needs to be addressed. Any idea what maybe the problem? I appreciate any help you can provide.