conspicuous by one's (its) absence

Noticeable by the very fact of not being there. The idea was expressed very early on by the Roman historian Tacitus, in recording the absence of Junia’s brother, Brutus, and her husband, Cassius, at her funeral procession. The phrase became popular in the nineteenth century, and continued to be applied often to political matters, such as the absence of certain provisions in a law, or the absence of political leaders on certain important occasions.