The Brass Key
Between the car key and the house key.
The small brass key opens the bottom drawer of Tom's desk. For eleven years it has ridden on his key ring between the car key and the house key, and he always hangs the ring on the hook by the front door with the brass key in the middle.
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After you finish
One Tuesday morning Tom finds his keys hung with the house key in the middle instead, and he knows Claire has been in the drawer (Chapter 25). When he leaves in December, he works the brass key off his ring and hangs it on the hook by itself. Claire dusts around it. When the movers come for the desk, she tells them there's no key.