Do you have pictures of that?
I believe that's mostly an urban myth, since the IR radiation from the body does not penetrate clothing to any significant amount. It's the temperature of the clothing that you see. I worked on a military FLIR system which operated in the the IR region around 10 microns which is most sensitive to objects near room temperature, and a view of a person only showed their clothing and exposed skin temperature. The clothing was, of course, warmer where it directly contacted the body, but no details beneath the clothing were visible.
One interesting observation was that you could see the inside of a person's nostrils get lighter and darker as the person breathed in and out.