A two-minute Halloween mystery with a fair clue

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Main story

After closing, the library felt larger than it did in the day. The lamps over the returns shelf were still on. Rain had stopped before lockup, but the air by the side passage still smelled wet. Mira locked the front desk and found one book that looked newly handled. It was a thin guide to city trees. Inside the cover sat a fresh leaf, green and damp, with a bead of water on the stem. A short grey thread was caught on that stem. Mira had seen the same grey on the dust cloth hanging from Jules's cart when Jules rolled past the desk at the start of the evening round.

The due slip showed a name, Samir, and a return through the after-hours slot. Mira almost called it a borrower's joke. Then she looked again at the thread and the water. The slot door was dry. The mat inside the lobby was dry. Only the side passage, where Jules parked the cart, still showed a wet print from the courtyard. The reading room beyond the shelf was already dark.

Jules was stacking chairs by the reading tables. Mira waited until the last chair was down, then held up the leaf, not as a charge, but as a question. Jules nodded at the cloth on the cart. "It fell on the bin while I brought the cart in. I put it in the book so you would see it before the dryer air turned it brown. Samir's book was already there. I should have left a note."

The leaf had never been a message from the borrower. It was the cleaner's quiet way of saying the courtyard tree had started to drop. The grey thread on the damp stem had been the proof the whole time.

Clue and solution
The early clue is the short grey thread caught on the fresh, damp leaf, together with Mira having already seen that same grey on the dust cloth on Jules's cart. The dry returns slot and the wet print by the cart only support that same clue. No new fact is added here. Jules put the leaf in Samir's already returned book. It was not a message from Samir.

Alternate ending, same clue
Jules still handled the leaf: the grey thread is still from the dust cloth, and the leaf is still damp from the courtyard. The intention changes. Jules was holding the leaf against a picture in the tree guide to see if the courtyard tree matched, then forgot to take the leaf back out. Samir still did not leave it. There is no new character and no magic.

Approximate main-story word count: 296. A two-minute reading time is approximate. Do a read-aloud check. Do not treat the runtime as exact. This is fiction, not a report of a real event.