undetectable
🇺🇸
/ˌʌndɪˈtektəbl/
🇬🇧
/ˌʌndɪˈtektəbl/
so small or hidden that it cannot be noticed or found
The spy's hidden camera was undetectable to the security team.
间谍隐藏的摄像头对保安团队来说是不可检测的。