Tried to view the PDF of the schematic, and...
Sorry, you don't have permission for that!
Never heard a piezo alarm, but if there's a true on-off switch to disable the buzzer, I'd have never thought to look for it or expect a buzzer at all. Bought the unit nearly-new-but-used. Will look around the lid. Doesn't help that the unit is black, so stuff in the back isn't really easy to see.
Thanks for posting.