"But aren't the two rulings inconsistent? I.e., don't you release a heat signature into public air, just as you do a scent? If so, why is a sniff okay but a thermal image not okay?"
I think the difference is that the thermal imaging cases involved looking into homes and courts have ruled that people have a greater expectation of privacy in their homes than in their vehicles. Also, these thermal imaging devices can actually see the thermal images of people in the homes, doing whatever people might be doing in their homes that they might not want others watching them do. It is a greater invasion of privacy.
On the contrary, thermal imagers merely read the temperature of the outside surface of a house. Both wood and glass are opaque to far IR. The reason for the ruling was not that cops could see people doing things inside their houses, but rather that the specialized technology crossed the line too far beyond the "plain sight" arguments offered by the cops. (Presumably when everyone has thermal imagers in their cell phones, this reason will vanish. It has already vanished for radar speed guns, the practice of using a formerly classified military technology to enhance revenue at the municipal level.)
Incidentally, submillimeter wave radars see through wood, clothing and drywall ... those things aren't thermal imagers. And SCOTUS hasn't ruled on their use, either.
From the present ruling, I'd expect the court to be all for SMMW radar, so long as the suspect was actually guilty - and therefore have standing to appear before SCOTUS in the first place.