select * from v$parameter where name like 'undo%'
Please post results.

It'll be retained till undo_retention time has passed and it becomes expired. Your undo_retention is probably a huge number of seconds.

Here's some good info:
DROP TABLESPACE undo02 INCLUDING CONTENTS AND DATAFILES;
· The Undo tablespace to be dropped cannot be in use.
· The clause INCLUDING CONTENTS AND DATAFILES causes the contents (segments) and datafiles at the operating system level to be deleted.
· If it is active, you must switch to a new Undo tablespace and drop the old one only after all current transactions are complete.
· The following query will display any active transactions. The PENDING OFFLINE status indicates that the Undo segment within the Undo tablespace has active transactions. There are no active transactions when the query returns no rows.

SELECT a.name, b.status
FROM v$rollname a, v$rollstat b
WHERE a.name IN (SELECT segment_name
FROM dba_segments
WHERE tablespace_name = 'UNDO01')
AND a.usn = b.usn;

NAME STATUS
------------------ ----------------
_SYSSMU2$ PENDING OFFLINE