just 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.