The LED is integrated in the sensor housing, below the sensor dome.
• Orange blinking: Sensor is ready to be connected to Hue bridge. (stops automatically after 20 minutes)
• Green blinking: Sensor identifies itself (e.g. when renaming using the Philips Hue app)
• Short orange flash at detection*: Battery is low
• Short red flash at detection*: Battery is (almost) empty
• Long red flash at detection*: Sensor cannot reach bridge (if this happens often, please consider a Zigbee channel change using the Philips Hue app (Settings > Hue bridges > Zigbee > Change Channel)
*Only shown at first detection after at least 15 seconds without motion