CIBSE Gain results can often display Solar Gains at night.
The CIBSE Gain calculation considers both an instantaneous solar component and mean solar component. The mean solar component consists of solar gains that are re-radiated from surfaces having being absorbed as gains during the sunshine hours.
The Cooling Plant Operation in Building Properties (accessed from project tree as in image below) determines the hours that these gains are considered.
Therefore, if you have set your Cooling Plant Operation to 24 hours a day, the gains for each of those hours will be displayed in your results.