To expand on what Sophist just posted, there's a standard for probable cause that states that objects in "plain sight" - i.e. observable without conducting a search - can become probable cause for search.
For instance if an officer pulls you over and observes a water bong strapped into the passenger seat, or the barrel of a concealed weapon poking out from under a chair (in a state where that is illegal), or anything that would violate a parole if you're one one, he has cause to search your vehicle, even though he didn't have consent to search or cause to look for such an object.
There's still discussion going on whether use of thermal imaging (which is used to detect cannabis incubators among other things) constitutes a search; the same would apply to this technology. If it doesn't constitute a search, then it may fall under the "plain sight" rule. Note that the standard is higher to enter someone's home, but a plain sight observation might be enough to justify a search warrant - and in any case cops have a habit of trying to obtain consent to search, at which point whether or not they had cause is irrelivent. If a cop says, "Do you mind if I look around" and you consent he doesn't need cause; I know people that have been busted like this a bunch of times because they're convinced if they say no that will just further incriminate them (which is untrue, but really what they maybe aught to do is stop breaking the law ;/).