All 3 Uses

covert
in
I Am a Seal Team Six Warrior
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I came down off the roof and covertly went next door.†
p. 95.6 *
In Second Phase, Land Warfare, we learned covert infiltrations, sentry removal, handling agents/guides, gathering intelligence, snatching the enemy, performing searches, handling prisoners, shooting, blowing stuff up, etc. As a child, I learned attention to detail—making sure that not one single pecan remained on the ground when my dad came home saved my butt from getting whipped.†
p. 34.3
Our procedures were covert enough that an enemy wouldn't know a signal was being given, and although we kept the procedures simple enough for our assets to remember, we spent hours reviewing the procedures with them.†
p. 93.6

Definitions:

(1) (covert as in: a covert plan)
done in a secret or hidden way
(2) (meaning too rare to warrant focus)
Much more rarely, covert is used as a noun to describe a hidden or sheltered place. Coverts can refer to the small feathers that cover the base of a bird's flight feathers.