Today I looked at it again and didn't notice any smoke coming from underneath the car and there seemed to be suffuicient exhaust coming from the rear tailpipe. I happened to be near my mechanic and he also looked and didn't see anything and suggested that sometimes there is condensation in the muffler and that can cause some steam/smoke to appear. I am going to watch it carefully for a while and see if I see it again.

Thanks for all of the suggestions!

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1994 E320 Cab (mine)
2001 Chevy Suburban (wife's)
1995 BMW M3 (son's)
1989 Volvo 740 GL (extra car--for sale)