This article refers to the Go Direct^® Energy Sensor (GDX-NRG).
The Vernier Energy Sensor (VES-BTA) does not have a warning LED.

The warning LED (next to a triangle containing an exclamation point) is meant to come on when there is a measurable potential difference (voltage) between the terminals but no measurable current, as when there is no load connected. It indicates that the load switch is in the External Load position but no external load is connected.

The LED works well with wind turbines. However, when using the KidWind 2V/400mA Solar Panel (KW-SP2V) as a power source and with no load connected, the warning LED may fail to flash. This is due to the solar panel voltage matching the internal voltage of the sensor, a voltage value which the sensor is programmed to ignore. If you have no load connected and you attempt to collect data from a solar panel with the Go Direct Energy Sensor, you will see that all measured values are zero. To get measurable values, connect a load.

Related articles:
Why do I need a load when measuring energy with an energy sensor?