This character masturbates discreetly in the presence of (a) third-person audience(s) unaware of the act. e.g. caressing one's own genitals under the table, masturbate where one is invisible, etc.

Unlike exhibitionism which draws sexual pleasure by exposing the act to an audience, discreet masturbation focuses on masturbating publicly but discreetly where the audience knows nothing of what really is going on.