The police planted a listening device in the suspect's room.
We checked whether a bug had been planted in the conference room.
盗聴 is the act of wiretapping or eavesdropping, while 盗聴器 is the physical device used for it.
Standard kanji spelling; the most common written form.
Variant kanji spelling using 機 (machine) instead of 器 (device); less common but still encountered.
Compound of 盗聴 (eavesdropping) and 器 (device). The spelling 盗聴機 uses 機 (machine) instead, with no difference in meaning.