If you issue the command DROP DATABASE for a SQL Server database that is offline, the associated MDF and LDF files will not be deleted.
In a related note, if you take a database offline using
sp_dboption 'mydb','offline','true'
you may get this error message when you attempt to bring it online:
Device activation error. The physical file name 'c:\blah\mydb.ldf' may be incorrect.
Use the following command instead:
alter database mydb set offline with rollback immediate
The former appears does not appear to offline the database files cleanly.