Drop Index Tablespace - File Is Non-empty

db 10.2.0.4
AIX 5.2

I am trying to drop a index tablespace I moved all indexes to new tablespace there is no indexes in that tablespace even any objects but still it is not dropping.it is giving error as below.

SQL> drop tablespace ret_index including contents;
drop tablespace ret_index including contents
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-02429: cannot drop index used for enforcement of unique/primary key

SQL> alter tablespace ret_index drop datafile '/path/index3.dbf';
alter tablespace ret_index drop datafile '/path/ret_index3.dbf'
*
ERROR at line 1:
ORA-03262: the file is non-empty

try to resize

SQL> alter database datafile 4 resize 100m;
alter database datafile 4 resize 100m
*
ERROR at line 1:
ORA-03297: file contains used data beyond requested RESIZE value

Tablespace size is 280gb