Both Uses of

discreet
in
Red: The Heroic Rescue

Fortier hadn't grasped the true potential of the right biological weapon until he watched one of those nations discreetly use an agent of Svensson's against the Americans.†
Chpt 11 *

discreetly = in a manner that is unlikely to attract attention or cause embarrassment

He'd spent most of the last two days scouring the electronic highways and making discreet phone calls in his attempt to piece this puzzle together, and now that it was together, he wasn't sure his effort had been a good idea.†
Chpt 15

Definitions:

(1) (discreet as in: discreet--not showy or gossipy)
trustworthy with secrets or subtle (not attracting attention)

This sense of discreet sometimes indicates that something is small (so as not to attract attention) as in "discreet earrings."
(2) (discreet as in: discreet--not foolish)
sensible and careful; possibly having good manners

This sense of discreet is more commonly seen in classic literature than in modern writing. There is may also refer to having the judgment to exercise good manners.