Hello Manish,
The auditing information depends on your auditing policy. In other words: if you're not auditing DROP / DELETE statements then you won't see those logged on your audit logs. That's probably why this is not on your audit log. Chances someone did what was done and cleared the audit log maybe?
And yes, you're right. DDL won't show up in SQL Plan Cache while DML will. So both M_SQL_PLAN_CACHE and _SYS_STATISTICS.HOST_SQL_PLAN_CACHE wont have that info.
BRs,
Lucas de Oliveira