On this page of the Disco Elysium game guide, you will find a detailed walkthrough of the Read the watermarks task.
In Disco Elysium, the task Read the watermarks requires you to read the halogen watermarks in your ledger, the same one you found in a locked trash container at the back of the Whirling-In-Rags building.
During the conversation with Kim, he will mention that RCM car lights are designed to reveal halogen watermarks.
Kim's car is parked in front of the hotel building. Approach it and turn on the lights. Next, stand in front of the car and once a sphere appears over Harry's head, click on it to initiate a dialog. This will allow you to learn some new information about the case and Harry's past.