He wore a hidden microphone to secretly tape-record conversations.

他戴了一个窃听器秘密录下了谈话。

《柯林斯英汉双解大词典》

A microphone is cleverly hidden in the seam of the battery cover, above and to the right of the camera pod, and the speaker is located on the signature chin at the bottom of the device.

副麦克风的设计很聪明，被藏在了电池盖与机体的上边缘的接缝下面，摄像头右上面的那个小孔就是。 外放则设置在了机器的底部。

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Instead of overcoming my fear of talking to strangers and doing interviews, I decided to wear a wire - a hidden microphone under my shirt - and for the next three years I recorded my life in secret.

BBC: Why I secretly recorded my life