Sounds like another database may have been created that is referencing the files.

In HP-UX, I can do an fuser on the file and see which process has it open. For example.

bitbom # fuser system01.dbf
system01.dbf: 7447o 7457o 7445o 7453o 7471o 7451o

bitbom # ps -ef|grep 7447
oracle 7447 1 9 10:57:52 ? 0:05 ora_lgwr_DLAW7
root 17991 17889 15 15:55:07 ttyp1 0:00 grep 7447

Note that each process has an 'o', this just indicates that the process nnnn has the file open. But it tells me which process has it open.