In his Slate piece from 2002, Eugene argues that checking radiation levels to avoid a nuclear attack is "just different" from using an infrared device in a routine criminal case. He writes:

  Sure, normally the Fourth Amendment applies equally to all serious crimes, and that's normally right. But finding dirty bombs must simply be different from fighting normal crime. Searches for weapons of mass destruction can't be treated like searches for marijuana-growing devices or even for murder weapons.
  The Fourth Amendment, by its terms, only bans "unreasonable searches and seizures"—and it cannot be unreasonable to examine homes with Geiger counters in order to prevent a city from being rendered uninhabitable by an enemy bombing. Protecting people's privacy is important, and so is constraining government power. But sometimes we need extraordinary government power to protect against extraordinary threat.

  I just thought I would add that there is a Fourth Amendment doctrine that recognizes Eugene's intuition: the "special needs" exception. The idea behind the "special needs" exception is that the government has lots of legitimate interests beyond collecting evidence in criminal cases. When government agents are pursuing those other interests, the warrant requirement is relaxed and the overall requirement is reasonableness. There are a bunch of Supreme Court cases on this doctrine, starting (I believe) with Camara v. Municipal Court in 1967 and receiving more formal attention in cases like O'Connor v. Ortega.