When Val had earlier scrutinized Lorel's room from the outside, his keen observation had led him to the discovery of a diary carefully stashed inside the bedside table.
Given that the room was Lorel's personal space, it was logical to conclude that the diary belonged to him.
As for why he was keeping a diary, it wasn't too hard to come to a conclusion.
In this era, the absence of mobile phones and similar technology meant that people often resorted to more traditional means of recording and remembering important information. Some even treated it as their hobby.