Did I not take it up?
Picking his nose at the end of Proteus, Stephen reaches for "My handkerchief. He threw it. I remember. Did I not take it up? / His hand groped vainly in his pockets. No, I didn't. Better buy one." The narrative here is artful not only in not representing Stephen's nose-picking (he appears largely oblivious to his body, and his urination a minute earlier has also gone unreported), but also in not representing, much earlier, his picking up of the handkerchief that Mulligan ("He") threw (because Stephen did not do so).
In Telemachus Mulligan borrows Stephen's handkerchief to wipe his razor. When he takes off his dressing gown quite some time later, he empties the contents of the pockets onto a table and points them out to Stephen: "— There's your snotrag, he said." Rummaging through his trunk to find a clean handkerchief, he comes across something else that belongs to Stephen: "A limp black missile flew out of his talking hands. / And there's your Latin quarter hat, he said. / Stephen picked it up and put it on." It is clear in context that Stephen has noticed and retrieved the hat but has not done the same with the handkerchief, perhaps because he was distracted by the hat. Or rather it would be clear to any reader who had the supremely myopic gaze to notice such things while navigating the first chapter. Not until the end of the third chapter does Joyce suggest that it would have been a good idea to be paying such close attention.
Ulysses, an edifice built of precise details, is as interesting for those that it omits as for those that it includes. The following chapter, Calypso, affords several examples, starting with the minor curiosity that the Blooms live at a precise address which is never mentioned in the chapter, building to the more interesting fact that Bloom cannot remember what he has done with his hat, and concluding with the major retrospective mystery that the 8-9 hour must have ended with an unnarrated conversation in which Molly disclosed the time of Boylan's visit.