conspicuous by its absence

Also, conspicuous by one's absence. Glaringly obvious by the fact of not being there. For example, One agenda item concerning publicity is conspicuous by its absence, or The bride's father was conspicuous by his absence. The idea is ancient; it was expressed by the Roman writer Tacitus, concerning the absence of Junia's brother and husband at her funeral procession. [Mid-1800s]

Very obvious through nonattendance. This oxymoron, which goes back to ancient Rome, applies to people or objects that attracted attention because they were expected to be present but weren't. An example would be a close relative who either wasn't invited or chose not to attend a family function. Some literary commentators contend that the phrase has become a cliché, but it's now used so rarely, you may—although at your peril—claim its wit to be your own.