Holden identifies the impossible sphere as a bullet from the force that deactivated the protomolecule on Ilus and erased the gate civilization.
The passenger remains fixed aboard the Heart of the Tempest, visible but physically and instrumentally absent. When Santiago Singh shows its image to James Holden, Holden recognizes the same impossible light-and-dark artifact encountered on Ilus and calls it a bullet. On that world it defined a zone that killed active protomolecule systems and temporarily shut down Elvi Okoye as she crossed it.
Holden connects the bullet to his vision from the ring station: the gate builders watched whole systems go dark, destroyed solar systems in an attempt to contain the attack, and still vanished. The sphere is therefore not simply a strange side effect or likely human weapon. Its appearance after the Tempest's new technology is used in Sol suggests that the ancient destroyer has noticed or answered Laconia.
A floating sphere of simultaneous light and darkness, visually undeniable yet perspective-breaking, invisible to instruments and intangible to matter.
The passenger is the returning signature of the force that killed the gate builders and the first evidence that Laconia's expansion has awakened it.