A boarding pass turned up once in a returned copy of a gardening manual, dated eleven years prior, a flight from a city the borrower had, according to the library's own records, never lived in. This is the ordinary condition of the forgotten bookmark: a receipt, a photograph, a pressed leaf gone the color and texture of a moth's wing, a child's drawing folded in quarters, occasionally a real bookmark laminated and clearly meant for the purpose, ignored in favor of whatever was closest at hand when the reader set the book down and did not pick it up again.
Staff who reshelve for a living develop an informal taxonomy of these objects without ever naming it as such: the grocery list, the appointment card, the photograph too personal to leave behind and too anonymous to return. Most are removed and discarded during processing. A rarer subset gets pinned to a corkboard near circulation under a hand-lettered sign asking whether anyone recognizes the handwriting, and almost nobody ever does.
What the bookmark records, more precisely than any date stamp, is the exact page at which a reader's attention left the book and went somewhere else entirely, though the object cannot say where. Sometimes that departure is permanent. Sometimes the same reader returns eleven years later for an unrelated title and forgets a boarding pass in that one too, the interruption simply changing address, still unresolved, still holding a place for a return that may or may not come.