You searched for: “undetectable
undetectable (adjective), more undetectable, most undetectable
1. Impossible to be noticed or perceived: The thief in the story wore a magic cloak that made him undetectable or invisible so he wouldn't be caught!

A color, for example is undetectable to the touch!
2. Hardly able to be seen or discerned: The fall in the temperature outside was undetectable and could hardly be felt.

This entry is located in the following units: -able (page 39) teg-, tecto-, tect- (page 2)