Leo says that if McAfee has told Phillip there's an intruder, it's probably a false positive. Leo doesn't recommend McAfee because it's just a step below a virus itself. The Chatroom says there's a forum post on McAfee on this issue and it shows it's likely bogus.
Leo suggests password protecting that router to prevent someone from getting on his WiFi without his knowledge or permission. Use WPA2 encryption. Leo predicts once Phillip has done this, he'll never see the "intruder" again. If it's an ISP router, Phillip will need to call them to find out how to secure it.