When Nico heard Percy say that his sister was dead, he looked behind him.
When he discovered his dad was none other than Hades, he looked behind him again.
When he gambled with Minos and looked for Deadalus, he looked behind him for a third time.
For months Nico's expected to see Bianca just standing right there, to see her flash a kind smile at him.
He had fooled himself into thinking that she'd just magically come back, he didn't see her actually die, honestly.
No, he looked behind him.
Bianca wasn't there.