The main RuneScape Wiki has an article on: Little key
The Old School RuneScape Wiki also has an article on Little key.

The Little key is an item used during the Plague City quest to open Elena's cell. It is obtained by searching the barrel nearest to the stairs in the basement of the house in West Ardougne where Elena is currently being held. You cannot enter the house until you have obtained a Warrant that is given to the player by Bravek, after which it is not needed to access the house.

Players who wish to obtain multiple keys may utilize the Drop trick. However, they must not have unlocked the gate. Unlocking the gate will make re-obtaining the key impossible.

After opening the gate, the key is no longer needed and can be opened without the key.

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