Hi Michael,
I think Andy mentioned using tcpdump in an earlier post to the mailing list:
I still have the command in a text file: tcpdump -vpni eth0 'host
212.13.19x.y'
That should show you what is still using the old IP. If you're confident
that there isn't anything there of importance you can try removing the IP
now and see if anything fails, that way you can easily re-add it until you
fix the problem.
Daniel