All 3 Uses

discreet
in
The Glass Menagerie
(Edited)

The only way to find out about those things is to make discreet inquiries at the proper moment.
Scene 5 *

discreet = unobtrusively (in a manner that doesn't attract attention)
That is the way such things are discreetly handled to keep a young woman from making a tragic mistake!
Scene 5

discreetly = in a manner that is unlikely to attract attention or cause embarrassment
He carefully shapes the brim and the crown to give a discreetly dashing effect.
Scene 7

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.
(3) (meaning too rare to warrant focus)