If they were overheating, there would be liquid and smoke emmanating much as you have described. Do you remember any smell? Coolant has a very distinctive smell. The steam that comes from an overheating radiator will quickly dissipate like the cloud you described, and if it came up from under the hood, it could look like it was coming from the window. Could the liquid you saw be coming from the front where the radiator overflow is?
Since you do have the tag numbers, it would be easy for the feds to track down and confirm/deny. Call them if you like. They won't hurt you.
I did not smell anything. I had my windows rolled up though. The fluid appeared to be coming from underneath dripping heavily for about 5-10 seconds then stopping completely, then starting again 30-45 seconds later. The white cloud seemed to eject from the passenger window, I can't be certain however that it wasn't the radiator, and the wind blew the smoke to the east side of the cab and made it appear to be coming from the passenger window. But, I guess, better safe than sorry.
I guarantee this however. If the feds do track this person down from the plate number I gave them, there is NO WAY the driver does not know about that cloud of smoke. It was massive. I wish folks here who are suggesting cigarette or cigar smoke could have seen it. I come from a family of smokers and there were sometimes 3 of them in a car. I do know the difference.