Well, if there is no running application claiming it, my first suggestion would be to run something like chkrootkit and make sure there isn't a rootkit installed.
Have you tried connecting to it with telnet or something similar to see if it announces itself?
You could also try runing tcpdump on it to see if its passing any traffic.
You could add an IPtables ruleset to block both outgoing and incoming connections on that port as well in the meantime, see what breaks when you do it
Normally, I think that anything that doesn't have a appliciation claiming it, its up to some nafarious purpose tho.