The Seeds of Non-identification (正体不明のタネ) also known as Seeds of Unknown Form were small snake-like flying objects with no determined form. Because of that, when people saw these, what they saw depended on their own experience and knowledge.
To Reimu and Marisa, these seeds looked like a block of wood flying in the sky (which aroused their suspicions, since they both thought that these had no business flying in the sky). Sanae thought it was a round device of the sort, similar to an unidentified flying object. It was later revealed that these seeds were attached to the fragments of the Tobikura that Murasa and other youkai were using to free Byakuren.