Venu,
If the alarm was trap based, then Spectrum determines which model to associate the trap to by the agent address in the trap. It could be any ip address configured on the device not just the ip address used to model the device in the Spectrum database. Spectrum reads the ipAddrTable and ipAddressTable tables and stores all known ip addresses for a model in the ip address list attribute for the model. When the trap is received, Spectrum reads the agent address of the trap and looks for any and all models in the Spectrum database that has that ip address associated with it.
If the other device is modeled in the Spectrum database then you can run the Devices -> By IP Address search to see if you can find it.
If it is not, there is nothing that I can think of to do in Spectrum to find this unmanaged device.
Regards,
Joe