RB941-2nD(hAP lite): 1 port showing active despite cable unplugged

I have RB941-2nD(hAP lite) . I have this router for internet connection and pptp VPN connection . after few days i am facing the issue that 1 of my port is showing connectivity light blinking up , without plugging the cable.

Can you please tell me the possible causes of occurring this and also the solution of the restoring???
Thanks in the advance.

Which port? What happens when you try to use that port?