This page of the Disco Elysium guide contains a detailed walkthrough of the side quest: Investigate apartment #10 for the cleaning lady.
In the Investigate apartment #10 for the cleaning lady quest in Disco Elysium, you have to check who is staying in room 10, which the cleaning lady says should be empty.
The quest can be started after examining the door of Room 10, when you ask the cleaning lady who lives there: Who lives in apartment #10? According to her, the room should be empty and she will ask you to check it out.
Interact with the door to Room 10, then convince the person inside to open it. Inside, you will meet Marielle Charpentier, a Marinaise Realy Associates agent. After talking to Marielle, go back to the cleaning lady and inform her that you've met a real estate agent preparing a room for the new tenants.