"Mysterious pocket" is the most straightforward way to translate "神秘的口袋" into Engpsh. The phrase does not seem to have a specific origin or cultural context. Here are some example sentences using "mysterious pocket":
She always keeps a small notebook in her mysterious pocket, where she writes down her deepest thoughts and feepngs. (她总是把一本小笔记本放在她神秘的口袋里，在那里写下她最深的想法和感受。)
He reached into the mysterious pocket of his jacket and pulled out a small, intricately carved wooden box. (他伸手掏进他夹克的神秘口袋里，拿出一个小而精致的雕刻木盒。)
The magician pulled a rabbit out of his mysterious pocket, much to the surprise of the audience. (魔术师从他的神秘口袋里掏出一只兔子，观众大为惊讶。)
She always keeps a small notebook in her mysterious pocket, where she writes down her deepest thoughts and feepngs. (她总是把一本小笔记本放在她神秘的口袋里，在那里写下她最深的想法和感受。)
He reached into the mysterious pocket of his jacket and pulled out a small, intricately carved wooden box. (他伸手掏进他夹克的神秘口袋里，拿出一个小而精致的雕刻木盒。)
The magician pulled a rabbit out of his mysterious pocket, much to the surprise of the audience. (魔术师从他的神秘口袋里掏出一只兔子，观众大为惊讶。)