Statement from Randy here:
Update on 787 event in Boston
The smoke was later traced to the battery used to start the auxiliary power unit.
We can’t talk about any specific details while the investigation is ongoing. But I can tell you that nothing we’ve seen in this case indicates a relationship to any previous 787 power system events, which involved power panel faults elsewhere in the aft electrical equipment bay.
Considering the type of battery the lack of damage to the surroundings is reassuring (not that I'd like a mouthful of that smoke).