A flashing network LED indicates network traffic on the link, this is not necessarily access to the NAS - most networks have some level of broadcast activity which will cause the network LED to flash.
The first thing to determine is if the NAS is hibernated or not - assuming the power LED is enabled, it will be dim if the NAS is hibernated, and on if it is not - if the power LED is always on with the unit configured to hibernate, the next step is to determine if it is external access or an internal process keeping it from hibernating - unplug the network cable - if it hibernates, external access is keeping the NAS awake, if it doesn't it is an internal process.