User X put some privilege ,such as create table privilege, grant to user Y,
SQL>grant create table to Y with admin option

Then user Y secretly put create table privilege to grant to user Z
SQL>grant create table to Z

My question is how user X can find user Y put this privilege secretly to user Z?

Thanks

DBA_SYS_PRIVS :
System privileges granted to users and roles.

or try this
1.in INIT.ORA
AUDIT_TRIAL=DB
2.then
audit DBA by <user>/
3.then
select ,,, from DBA_AUDIT_OBJECT