Network traffic activity?
not sure, it's not really consistent. It seems to do so at boot and for about an hour after boot. After that it's just permanently on for several hours until it starts blinking again for a couple of minutes. Streaming something from my NAS doesn't seem to make it blink. To check if the LED is perhaps dying I've booted the box into stock android where the LED stayed permanently on.
I've checked in /proc if there's anything related to it but I'm not really sure what to look for as it's one big mess with subdirectories.