Key to Dijkstra's treasury

Key to Dijkstra's treasury is a quest item in. During the main quest, Count Reuven's Treasure, after Caleb Menge is dealt with, Triss will find this key on his body and give to Geralt to give to Dijkstra. However, if Menge wasn't asked about the treasure, the key will ultimately be useless. If he was asked about it though, Geralt learns it's the key to a vault in a Koviri bank and passes on this information to Dijkstra.