Lt. Mosley's monogrammed key ring was foolishly dropped at the scene of Sheriff Truarts murder in the mission; Blackmail.
Garrett, who initially meant to confront Truart with the mechanist recording from First City Bank and Trust, found the key ring and decided to follow Mosley in order to find out her motive in the killing, only to discover it was someone else, with her cooperation[1].
She admitted it was foolish to put the key for the assassin of Truart to use on a key ring with her emblem on it but despite fears it had been dropped at the scene, no-one found it (because Garrett had already taken it)