Andrew had no idea that the wax mark on the letter was the one used by the Castal family, but since the letter had been sent by Johan there were only two possible people who would send it.
The first was Johan, but Andrew didn't think Johan would go to the trouble of sending him a sealed letter to tell him something when he can come to the Heavenly Palace like he has on previous occasions.
Therefore, there was only the option of Baron Castal, who had previously told Andrew that he would communicate the situation of his son later through Johan, and since it was the latter who sent the letter without leaving any message, Andrew assumed that it was the baron and after opening the letter he corroborated his theory.
