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."

Thank god for that.

There's already precedent about this involving radiated IR, which ruled that police may not use such tools without a warrant, as it violates what people normally consider to be private information.

It's possible that different rulings will come in the future, but you have people defending drug dealers to thank for essentially ruling that this requires a warrant at present.

Active versus passive is less of an issue here than a 'reasonable expectation of privacy'. Regardless of whether you're transmitting energy into a premises, you're still 'looking inside'.

It's worth noting that a normal human without engineering training is aware of silhouettes or even if the window is open - if they do things in plain view, there is onus on them. It takes specialised knowledge to even know this hi-tech stuff is possible for the most part, so you can't really equate passive collection of undetectable-by-human radiation to things like windows and silhouettes, when it comes to the average person.