In the summer of 1944, he was at the Natzweiler-Struthof concentration camp in Alsace with another SOE agent, Brian Stonehouse. At the camp he witnessed the arrival of four other female SOE agents, Andrée Borrel, Vera Leigh, Diana Rowden, and Sonya Olschanezky, who were all executed and disposed of in the crematorium in an attempt to make them disappear without a trace, under the programme of "Night and Fog". After the war, Guérisse and Stonehouse were able to testify at the Nazi war crimes trials as to the women's fate.