The Master Key can be found in the "Memory of a Lost City 8" chapter with an S-rank on Normal.
The Master Key to the Raccoon City underground research facility. In RE2 it was used to open the emergency route for the main elevator and to lock and unlock the guard room door.
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- Resident Evil: The Darkside Chronicles – File Location Guide. GameFAQs. Retrieved on 2020-03-22.