From the Comcast help site:
Why do some of the lights on my cable modem blink even when my PC is idle or turned off?
Many cable modems have an LED labeled Cable or Data. Often, this LED flashes or blinks even when your PC is idle or turned off. Legitimate network data (most often DHCP and ARP traffic) is generated by Comcast, causing this behavior. This traffic should not be cause for worry, and should not affect your bandwidth.