All 3 Uses of

discreet

in

To Kill a Mockingbird

...never take a check from a Delafield without a discreet call to the bank;
p. 149.5 *

discreet = inconspicuous (not attracting attention)

We ran to Tyndal's Hardware door— near enough, at the same time discreet.
p. 171.8

Mr. Raymond chuckled, not at all offended, and I tried to frame a discreet question: "Why do you do like you do?"
p. 228.2 *

discreet = polite (using good judgment)

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.