The small rusty key is a key of the Morton house which is recovered by Edward Carnby in Alone in the Dark: The New Nightmare.
The New Nightmare[]
Edward Carnby can find the little rusty key by solving the riddle linked to the bust of Howard Morton in the main hall of the villa. By entering the correct code at the base of the trunk (obtainable pushing the same in the mirror, so as to see the reflection code) will unlock its associated framework, opening it you will find the key. It allows Carnby to open the door of the second floor to the east, which is accessed from the attic.