Simply put: When the headphones are connected via Bluetooth and in the ear, the LEDs do not flash.
I did some tests with my headphones and found the following behaviour:

One headphone can work individually, so the flashing behaviour does not necessarily have to be the same for both headphones.
- If one headphone is switched on & connected via Bluetooth & the nutrient sensor is covered, the LED does not flash.
- If a headphone is switched on & connected via Bluetooth & the nutrient sensor is covered & playing music, the LED does not flash.
- If a headset is switched on & connected via Bluetooth & playing music but the nourishment sensor is not covered, the LED flashes approx. every 5 seconds.
- If a headphone is switched on & connected via Bluetooth but not playing music and the nourishment sensor is not covered, the LED flashes approx. every 5 seconds.
- If the headphones are not connected via Bluetooth, they will flash approximately every second.
- During the "Bluetooth connection process", a pair of headphones flashes several times a second.

No, they only flash when they are out of the case, but the proximity sensor on the headphone outputs does not detect anything. In other words, if you are looking for them because they are lying around somewhere, they will flash. As soon as they are in your ears, they stop flashing.