Very rare. I would imagine they had already been scoping out the house for some time.I was actually watching a marijuana documentary the other day where they got a search warrant for a heat signature given off by a 600w hps.
As true as this is....it sucks to say but it means literally nothing in terms of police and federales using them constantly to bust grow houses under manufactured premisejust found this from another post: Not my words but I've heard this before and I think it's true.
BTW, for U.S. people, the federales cannot use thermal imaging on you unless they already have a warrant for other reasons. The Supreme Court has already ruled that thermal imaging constitutes a search, and you can't search someone's home without a warrant. So they CAN thermally image you, but they need a reason to get a warrant first.