What is an indication that a refrigeration system may have lost refrigerant?

The presence of bubbles in the sight glass is a clear indication that a refrigeration system may have lost refrigerant. In a properly charged system, the sight glass should show a solid column of liquid refrigerant. If the refrigerant level is low due to a leak or loss, the sight glass will display bubbles, indicating that there is a mixture of liquid and vapor, which suggests that the liquid refrigerant is not adequately filling the system to operate efficiently.

In contrast, high discharge pressure may occur for a variety of reasons, including overcharging or a restriction in the system, and does not specifically indicate a loss of refrigerant. Increased compressor noise could suggest mechanical issues or improper lubrication rather than a refrigerant deficiency. Lastly, cold spots on the evaporator might indicate airflow issues or ice formation from moisture, which isn't a direct sign of refrigerant loss. Thus, observing bubbles in the sight glass is the most definitive sign of refrigerant loss, making it the correct choice in this scenario.