It's a bit of a bug. It seems to happen if there was an upstream glitch and it had to resync. You can try rebooting the modem and see if it clears up.
If that doesn't work, try holding in the recessed reset button on the rear for 30 seconds. I had the same thing with that modem. It didn't affect the performance / connectivity in any way, so I just ignored it. It's not a problem.
When following the steps provided by andyross & EG did this help with the blinking US light?
Ok, I have heard this is a bug. But since this is not a Comcast owned modem, I would have to advise you to contact the manufacturer for further assistance. I apologize for the inconvenience.
It still blinks.
Unless you are having problems, ignore it.