Why can't I drop this Oracle datafile - even though it seems empty?
Oracle here warn you from dropping datafile because it contain amount of data, to make sure the datafile is completely  empty please execute the following command

col tablespace_name format A30 heading tablespace
col file_name format A40
col tot_KB format 99,999,999
col tot_freeKB format  9,999,999
col Kb_max format  9,999,999

select f.tablespace_name
      ,f.file_name
      ,f.bytes/1024 tot_KB
      ,sum(nvl(s.bytes,0)/1024) tot_freeKB
      ,max(nvl(s.bytes,0)/1024) maxextent_freeKB
      ,( (sum(nvl(s.bytes,0)/1024)) / (f.bytes/1024) )*100 Pct_available
from   sys.dba_data_files f
      ,sys.dba_free_space s
where  f.file_id = s.file_id(+)
group by f.tablespace_name, f.file_name, f.bytes/1024
order by 1,2
;


if the datafile is completely empty, then you have one of the two option:

1) reboot the database ( sometime it help)

2) move all the data from one tablespace to another one, then drop the tablespace and rename the new one