I just checked with our data science team. So the short answer is: yes, it could happen that a detected device which is on for a prolonged period of time (like 24/7) could become part of what Sense sees as “Always On”. That said, our team thinks this is pretty unlikely for most real devices, since even a furnace which is providing a lot of heat is unlikely to be just running at a constant rate all the time. It may instead be considered part of “Unknown” if the detected device is not showing as on but Sense still considers it not part of “Always On”.
Note that if it does happen that Sense thinks the device is part of “Always On” it won’t break the existing furnace model, it would just make Sense unsure what the source of the power use is for that period in time.
As you know, Sense relies on machine learning algorithms, and certainly we expect this type of detection and logic to improve over time!