The white smoke could have come from a small oil leak, which, depending on how small it was, could have been an immediate get-on-the-ground NOW emergency or something they felt they could deal with until they found a better place to put down.
As far as hypothesis, there are reports of odd, unexplained flares and other things in the back country of Canada along what could have been the White Bird's flight path, at that time, if they were off course and the ground was largely fogged in, which I believe it was that day. Which is where the aircraft might have ended up. Which is an equally valid and testable hypothesis, based on the available information.
LTM,
Monty Fowler, TIGHAR No. 2189 EC