Sometime about A.D. 117, the Ninth Legion, which was stationed at Eburacum, where York now stands, marched north, and was never heard of again.
— Excerpt from the introduction
The Rosemary Sutcliff historical novel published in 1954 about a young Roman soldier named Marcus Flavius Aquila who goes on a quest to find the eagle standard of his father. It is loosely based on the mystery of the real Roman Ninth Legion, which disappears from the historical record after having last been mentioned as present in Scotland in the early 2nd century. This book sets up later novels featuring descendents of the protagonist. The 2011 film The Eagle is based upon this novel, but the earlier (1977) BBC TV series (titled The Eagle of the Ninth) is a more faithful adaption (perhaps because the 2011 film omits some characters and events for the sake of brevity).
This book (and the 1977 TV series) provide examples of:
Maybe Magic, Maybe Mundane: While Marcus is trying to steal the eagle, his lamp nearly goes out, but recovers when he prays to Mithras. Was it just a fault in the wick, or were supernatural forces at work?
Signature Item Clue: While staying with the people who took the titular eagle from his father's legion, Marcus removes the eagle during the night and hides it under the banks on the edge of a lake, for Esca to return in secret and retrieve days later, after the two of them have been followed and searched. Unfortunately, while doing so, Esca accidentally drops Marcus' distinctive ring-brooch, already "all but torn out" of Marcus' cloak through the roughness of the aforementioned search. When the natives find it days later, they give chase.
Unspoken Plan Guarantee: Marcus' plan to take the eagle without attracting suspicion is described in full, and is thwarted by Esca's dropping of the brooch. How the pair ultimately succeed is, by contrast, not described beforehand.