That kind of "search" might still need a warrant. For example, there was a court decision some years ago which determined that using IR cameras to detect marijuana grow lights inside a home requires a warrant, since people have an expectation of privacy that no one can see through their walls.
IANAL so I'm not sure whether expectations of privacy for letters are legally as strong as they are your own home, but it certainly seems possible...
IANAL so I'm not sure whether expectations of privacy for letters are legally as strong as they are your own home, but it certainly seems possible...