In the past, the maps you downloaded were NOT unlocked. It was the GPS or SD card specific key saved in the appropriate .UNL file that allowed the GPS to unlock the maps for use in your GPS.
The maps you are now downloading wich say "unlocked" means that their ability to be locked has been removed by some tool or process. To the GPS the maps appear "unlockable". There is no .UNL file required for an "unlocked" .IMG and if you've got a .UNL file hanging around your GPS you can delete it if you wish. It's doing nothing.