There is a good description of metox use and alarm and all that on pages 84-86 of the French version of U-Boote vol 2 vers la defaite by wolfgang frank (originally published in 1950 as Die wolfe und der admiral by jean noli) I am on hol so I can't translate it now.

This book was writted in 1948-50 I would guess and the author was close to the U-boat command and so the descriptions are fresh and interesting but of course lacking in detailed knowledge of allied side - hfdf and of course ultra.

Aside - on page 43 he describes everything relating to sigint/hfdf as .....
"Ultra-secret" !!!