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.
間諜隱藏的攝像頭對保安團隊來說是不可檢測的。