From memory, digikam leaves these files dotted around your system.You will find them on removable usb thumb drives. The best thing to do is to delete them from within digikam by going to the trash folder and selecting the option to delete the items from the disk (obviously usb drives must be connected). See https://docs.digikam.org/en/main_window/image_view.html#deleting-a-photograph

For each collection registered in the database, digiKam handles an internal trash-bin. Physically, the trash is located at the root album corresponding to the collection entry. It’s a hidden folder .dtrash storing deleted items in a subdirectory named files, and information about the deletion stored in another subdirectory named info with Json sidecars (.dtrashinfo). Deleting a file in the collection does not remove the file from the media, but moves the file into this container. This one is accessible from the left sidebar tab Albums as the last entry of the tree-view corresponding to a collection and is named Trash.

On the bottom of the trash-bin view, a series of buttons allows to restore files in collection or delete them permanently. These are:

1. Undo: to restore only the last entry in the trash-bin.
2. Restore: to restore selection of files from the trash-bin.
3. Delete: to remove permanently the items selection or all items from the trash-bin.

All operations processed on trash-bin items will be confirmed to the user.

As far as I know there is no setting to redirect where the deleted files are sent.