Final answer:
The provided texts do not contain information about a specific 'incredible treasure' Harry found in Sirius's bedroom; hence, the question cannot be answered with the given excerpts.
Step-by-step explanation:
While searching Sirius's bedroom, the "incredible treasure" that Harry found isn't explicitly outlined in the provided excerpts, nor is it directly related to Sirius's bedroom. The various texts describe different scenarios ranging from an object with dense material to descriptions of rooms and objects, but none precisely detail a treasure found in Sirius's bedroom related to J.K. Rowling's Harry Potter series. Therefore, the information given does not allow us to answer the question about the treasure Harry found in Sirius's bedroom with certainty.