Search the basement of the merchant’s nearby apartment

How do you complete "Search the basement of the merchant’s nearby apartment"?

This is step 4 of 4 in Line in the Sand, given by Celeste. It is completed on Buried City.

• Line in the Sand
• Buried City

What are the other objectives in Line in the Sand?

Line in the Sand has 4 objectives in total: Find out why the train never reached Marano Station, Find out who orchestrated the robbery, Find out where the thruster valves were stashed and Search the basement of the merchant’s nearby apartment.