The person was someone he'd noticed during the evening, a man who looked very similar to the portrait given to him by the princess. Although there were slight differences, he was convinced that this man was the one from the picture.
The man was dressed in low-key but elegant clothing, and he was an unfamiliar face in the Capital. Moreover, his outfit of dark green aligned with the description provided by the princess.
He had been planning to return to the Drunk Universe restaurant, but upon seeing this man, he became suspicious and decided to follow him in order to see where he might be going.
There was a curfew in the Capital recently, so most of the wealthy and refined gentlemen would be returning to their mansions. As such, he only needed to trace the man's whereabouts to find out which family he belonged to.
Or perhaps the man was an outsider staying at an inn.