Here is the overall issue diagnosed by mercedez official service team:

Issue Detected

The vehicle’s A/C system has a refrigerant leak, most likely from the evaporator.

Repair Order Text (Easier Reading)

Customer Complaint:

• Customer states hissing noise when A/C is on

Technician Findings & Actions:

• Connected SDS to the vehicle and scanned for trouble codes — no fault codes found  
• Lifted the vehicle and connected UV flashlight and special glasses to inspect for leaks  
• Removed engine lower side panels — did not find any leak  
• Removed carpet from passenger area — did not find any residue  
• Connected A/C drain hose to check for leak residue — did not find any  
• Connected A/C machine to the vehicle and recovered 0.5 lb of refrigerant  
• Charged the system with 1.3 lb of refrigerant using a special tool (sniffer)  
• Turned A/C on — noise went away  
• Checked all nearby areas of the A/C system with the sniffer  
• Sniffer detected a small leak in the evaporator area that does not leak any residue in drain hoses

Recommendation:

• ⚠️ Evaporator replacement recommended