Some networks (check with the network's help channel for accurate information) have a seenserv and supports !seen <ip>
Normally the !seen command (where available) only supports a nick, but I have seen (pardon the pun) people use an IP (in ident@host or NNN.NNN.NNN.NNN) format.
I'm going to guess that the OP doesn't have the nick the person was using last time, or that they suspect that they are using a different nick at this point.