B777 Captain reported during final approach ATC instructed them to go around when the aircraft was below 100 feet AGL due to traffic on the runway. Captain executed go around although ATC did not provide any go-around instructions.

Narrative

Operating a 777-200 from ZZZZ-ZZZ we had a generally routine flight. We expected and briefed the ILS-XXL approach and were subsequently told to expect Runway XXC and so rebriefed the ILS to that runway. We were cleared the visual approach to XXC and as we approached the airport we observed the preceding arrival land and clear the runway and another aircraft cross at the rollout end. Below 100 ft AGL the Tower broadcast excitedly; Aircraft X GO AROUND traffic on runway!" Without hesitation we executed the go around procedure and as we did so the Tower Controller made a remark that he had received an erroneous indication from his runway-occupied monitoring system and sounded just as surprised as we were at the occurrence. The controller did not issue us go-around instructions and that left us uncertain about whether we should follow the aircraft guidance; which was commanding a right turn in accordance with the ILS missed approach procedure; or continue runway heading. The Tower was also busy directing other traffic so I was unable to clarify our instructions until probably above 1000 ft AGL; and this delay and confusion led to a disruption of standard callouts and procedures. Positive control of flightpath was never in question but the safety margins were definitely reduced due to the lack of clarity of instructions. By the time we leveled at 3000 MSL we had regained the shared mental model and confirmed the aircraft was configured properly; the autoflight matched our desired flightpath and necessary checklists were completed."