Undetectable (adjective)

Not able to be discovered, recognized, or detected.

Origin:

Early 17th century: from un- (expressing negation) + detectable, from late latin detectabilis "capable of being detected".

Examples:

1. The virus became undetectable in his blood after the treatment.
2. The burglar left no traces that could make him undetectable.
3. The spy used an undetectable device to transmit secret messages.
4. The amount of drugs in her system was so small it was undetectable.
5. The soundproof walls were so well constructed that outside noise was undetectable.