Even Severus could guess, though he reconciled that with his own feelings and his sudden closeness to Remus and his not quite (but rather obvious) worry over the moon and it's transformation.

Sirius made himself available to Remus, but that was nothing new. The Marauders stuck close to each other on the day of the full moon. It was like survival instinct, the clustering. Even Evans was out of the picture for the day!