Or can the messages be written somewhere else than the udump or udump destinations?
Are you sure that you don't need to look in bdump?
Also did you check to see where bdump is pointing?

Code:
SQL> select name, value 
from v$parameter where name like '%dump%';

NAME                   VALUE
---------------------- --------------------------------
shadow_core_dump       partial
background_core_dump   partial
background_dump_dest   /u01/app/oracle/admin/t31a/bdump
user_dump_dest         /u01/app/oracle/admin/t31a/udump
max_dump_file_size     10240
core_dump_dest         /u01/app/oracle/admin/t31a/cdump