Quote from documentation:
detached
the entity has an associated identifier, but is no longer associated with a persistence context (usually because the persistence context was closed or the instance was evicted from the context)
Does it mean that hibernate keep references to all objects that have been ever persistence? But then it would cause memory lick.
Or it means that entity has an id that corresponds to an entity in a database then it would lead to two conclusions: 1) It can be checked only querying a database. 2) A detached entity can become transient if some one third would delete the entity from the database.