Is it possible that when you were working on the engine, some coolant spilled into the exhaust pipe? The reason I ask this is because we were doing a valve job on an engine and during the head removal, coolant found its way into the exhaust system. After reassembly, the engine exhaust reeked of anti-freeze and put out white steamy smoke for almost an hour before subsiding. Where you say that your oil looks clean and no apparent loss of coolant is evident, it could be a possibility. I hope that that is all it is. Good luck to you.
John R