Conspicuous by (one's) absence humorous idiom
People notice someone's absence because they are supposed to be there.
It's his own father's funeral. Of course he's conspicuous by his absence!
A: "Hmm, I wonder if anyone notices if I didn't show up to class today." B: "Dude, you are the class monitor. You will always be conspicuous by your absence."
To talk the main part at once, or avoid talking about what is not important
You should get straight to the point.