As TTFN said, if a person is standing in front of a window, AND you are using one of the cameras that detects in the 2-5 micron band, it MAY be possible to see them through the window. It is IMPOSSIBLE to see them otherwise (despite this being reported in the national press several times!).
Interestingly, some law enforcement agencies doing drug work ran into problems in the courts when using 2-5 micron cameras as it was deemed "invasion of privacy."
A new technology, micrometer imaging, does allow for some transmission of information through walls. This is currently being used only by the military.
Alien Visions, a great book by Dr. Austin Richards, delves into imaging technologies in all wavebands.
If I can be of further assistance, please feel free to contact me.
John Snell
Snell Infrared
