He hides his face and pretends not to hear: We are still talking here about the person making a show of fake seriousness. He pretends to be lost in thought, he pretends not to notice you or hear you. It is not that he literally hides his face; he simply will not look at you, probably wanting you to think he is so preoccupied that he doesn’t notice you. Contemporary English Version connects this line with the previous verse as follows:

• Some dishonest people
act like they are mourning,
hiding their faces from you
and pretending not to listen.

But where no one notices, he will forestall you: But where no one notices is literally “where he is not noticed.” Good News Translation says “when you least expect it,” and Contemporary English Version has “when no one is looking.” Forestall means “take advantage of, get the better of.” The idea is that this person who appears so lost in thought that he doesn’t see you or hear you is actually very much aware of you. He is putting you off guard in order to “take advantage of you when you least expect it” (Good News Translation).

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Sirach. (UBS Helps for Translators). New York: UBS, 2008. For this and other handbooks for translators see here.