Why does a red LED light appear in the evening?

The red LED light you see is an infrared light for night vision indicating that your night vision is currently ON.

Petcube comes with an infrared sensor capable of streaming clear black and white video in the pitch dark.

When the sensor detects a low level of ambient light, a Petcube camera will automatically activate the Night Vision mode.

By default, Night Vision is set to Auto mode, but you can default it to always On or Off going to Camera Settings -> Audio & Video