This is based on my understanding of how Windows works. When you plug a usb device in Windows the sequence of events is "New hardware detected", "installing drivers", "device is ready". When I plugged the light into my desktop, it blinked twice until Windows acknowledged that something had been plugged in.

This sequence is sometimes described as a "handshake" where the two devices "introduce" themselves to each other, in this case the USB device having to identify itself to the PC as to what device it is so that the PC knows which driver to load.

I only have the EE and PE, and no other Android device. Android can't be much different, though - if you plug in a usb device, Android has to figure out what you plugged in. My guesswork is that the brief light flicker I observed on my PC and the continuous light flicker on my eDGe are caused by the same thing. Could be the flicker on the eDGe is electrical and has nothing to do with the USB handshake. Or, it could be Android not getting the expected response from the USB light and (foolishly) continuing to ask for it.

Certainly battery level and on-AC/off-AC aren't definitive factors, as mine flickers when fully charged, nearly fully charged, and when partially discharged, and it happens if I'm plugged into AC or not. At other times it doesn't flicker in all the same situations.

Furthermore, I just woke up my PE and plugged in the USB light immediately, and it flickered a few times, then stopped (which again makes me deeply suspicious that it's the USB handshake). The PE is on Battery, with a battery level of about 40-45%.

Until Tech Support gives a clear answer, I can only offer educated guesses