Read his journal
From Create Your Own Story
First you break into his room, then you read his journal? What kind of friend are you? Apparently a nosy one.
You turn to the last page of his journal.
The officials in Oradea have been unhelpful. They insist that Walfreid's place of residence is his ancestral castle, even though it's clearly abandoned. I cited the information I found yesterday in the town records, but they won't budge. This afternoon I will make the trip up there to be absolutely sure. This case is turning out to be a handful. When I can tell the officials that I saw with my own eyes that the castle is run down and uninhabited, perhaps they will be more helpful.
So, Michael did visit the castle. You check the date of the entry. Yesterday: the day he quit the case.