Year to date, Sembcorp has gained 44.3% - the best performing stock among all 30 STI components.
The company, having split from loss-making subsidiary Sembcorp Marine, has been actively moving into renewable energy sectors.
In its response at 9.07pm, Sembcorp says it isn't aware of what might have caused the movement, and that it believes it is in compliance with applicable rules.
Sembcorp shares closed the day at $2.87, up 5.9%.
