undetectable
adj. 看不见的，察觉不出的；探测不到的
例句
The sound is virtually undetectable to the human ear.
这声音实际上是人耳难以听得到的。
《牛津词典》
Look, it says here that the noise from it should be undetectable by other people.
看，这里说的是它发出的声音应该不会被其他人察觉。
Worse still, such ploys are often undetectable.
更糟糕地是，这样的伎俩往往探测不到。
