Adjective

(er)

Hard to grasp; not obvious or easily understood; barely noticeable.

The difference is subtle , but you can hear it if you listen carefully.

(of a thing) Cleverly contrived.

(of a person or animal) Cunning, skillful.

insidious

* 1623 , , act iv, scene 4,

Thy age confirmed, proud, subtle , bloody, treacherous.

Tenuous; rarefied; of low density or thin consistency.

Adjective

(en adjective)

That has been moved out of sight. Invisible or unapparent.

*{{quote-book, year=1892, author=(James Yoxall)

, section=chapter 7, title= The Lonely Pyramid , passage=It was the Lost Oasis, the Oasis of the vision in the sand. […] Deep-hidden in the hollow, beneath the cliffs, it lay; and round it the happy verdure spread for many a rood.}}

*{{quote-magazine, date=2014-06-14, volume=411, issue=8891, magazine=(The Economist)

, title= It's a gas , passage=One of the hidden glories of Victorian engineering is proper drains. Isolating a city’s effluent and shipping it away in underground sewers has probably saved more lives than any medical procedure except vaccination.}}

Obscure.

That has been hidden.