You can see an example of this technique in the former headquarters of the STASI secret police, now STASI museum, in Berlin.
Apparently, suspects grabbed by the STASI to be interrogated would be seated in a special, though normal looking, chair. The interrogation room was very hot, causing them to sweat profusely.
When (if) the suspect was released, agents would remove a piece of cloth hidden in the lining of the chair, impregnated with his sweat, and seal it in glass jar.
If the need ever came to track a fleeing suspect, the cloth with his "signature scent" would be given to the dogs to sniff.

After the fall of East Germany, people found hundreds of such jars in storage, a true catalog of the scents of "deviants" and political dissidents.

Here's a picture of the jar in exposition.
http://andberlin.com/scent-jar/