Actually for Kerberos snooping, clients must have a direct layer 2 connection to the switch; that is, the connection must not cross a layer3 boundary. If the connection does cross a layer3 boundary, the gateway's MAC address gets associated with the identity which in return may cause this messages. As you said customer has already enabled id management on the ports as well the UPM script hence you can always look into the type of events being generated at the time of issue.
Basically Identity management events generate corresponding UPM events. The UPM events that are generated include:
● IDENTITY-DETECT
● IDENTITY-UNDETECT
● IDENTITY-ROLE-ASSOCIATE
● IDENTITY-ROLE-DISSOCIATE
But not sure if these log messages are still noticed because Kerberos identities will be cleared immediately if the Aging timer is not configured else it will be cleared after Aging timer is expired for this Kerberos identity.
Hope this helps..........