If you have ADSL (or similar), your ISP will constantly try to "see" your router (through whatever methods they use - ie, broadcasts, pings, etc), to check if it is alive, so that if it appears not to be there, they can give your IP address to someone else (obviously, only if you don't have a fixed IP). Just a thought.
Although, Subseven listens on 1243, so it might be worth checking your clients for infection. Why would someone be trying to connect to 1243 on a client machine, unless it already had S7 installed?