The room where the cabinets are kept
Customs evidence files are lootable cabinets that can be accessed after Rocking Out. Looting a cabinet requires 63 Thieving, and a successful attempt will grant 75 experience.
The cabinets are found on the top floor of Rock Island Prison, which can be accessed after completing the quest by speaking to the Customs Sergeant in the Rimmington Customs Office. To escape the cell, rediscover the crude lockpick by searching the bed. A Customs Officer will need to be killed to obtain a uniform, which will render other officers docile and grant access to the evidence files. After the uniform is obtained, no more combat is necessary.
Seventeen cabinets can be found on the top floor. They take approximately 275 game ticks (165 seconds) to resupply after being looted. Custom officers patrol the evidence room, but will wander aimlessly unless the customs officer uniform is unequipped.
The customs outfit and mithril scimitar will be lost when leaving the island. However, they can be disassembled.