What the Thief Forgot (盗賊の忘れ物, Touzoku no Wasuremono, lit. Thief's Forgotten Item) is an event in the first game.
Find and retrieve what the thief forgot.
Talk to the thief from the previous event who can now be found inside the chimney at the top of the Haunted Mansion. Talk to him in order to activate the event. He wants Tomba to go collect a item that he dropped in the Lava Caves.
The item lies on the platform where Tomba met the thief initially. Collect it and head back to the thief to clear the event.