I wouldn't consider the passive/active distinction the definition for 'invasive.' I'd consider thermal imaging from the outside of the home w/out a warrant 'invasive', but a thermal camera is just picking up the heat radiated by your body.
The article links to the relevant Supreme Court decision. They, in "Kyllo v. United States, 533 U.S. 27 (2001), held that the use of a thermal imaging, or FLIR, device from a public vantage point to monitor the radiation of heat from a person's home was a "search" within the meaning of the Fourth Amendment, and thus required a warrant." -http://en.wikipedia.org/wiki/Kyllo_v._United_States
The article goes on to say the Supreme Court "specifically noted that the rule would apply to radar-based systems that were then being developed."
