translation please

the context is one resistance chief speaking to another; about when he was a prisoner of the occupier germans; he's talking about the interrogation he went through, he's just said they didn't treat him too badly;

"Je crois qu'ils sentent quand la matiere n'est pas vulnerable. A moins qu'il y ait un signe sur les gens. Certains qui usent des precautions les plus poussees sont pris."

I think he's saying that they know when a person isn't vulnerable to torture; but the rest of it i can't figure out.

tia

isabelle