Some extensions use start scripts that are contained in the extensions (/usr/local/tce.installed/TCZ-NAME) to create symlinks or create copies of some files. These files will be orphaned after unmounting the tcz.
For extensions that do not contain a start script, an empty file is created in /usr/local/tce.installed .
Some unrelated scripts check for the existance of /usr/local/tce.installed/TCZ-NAME to check if a dependency is available (not sure if a dead symlink still returns 'true' if checked for by a script).
I guess unmounting can be safe for some, but not all extensions. Deleting the files which were created while the extension was loaded might be advisable, though. There is probably more.
--
Or, to 'get back' some memory:
sudo /usr/sbin/cache-clear
Not sure if it makes that much sense, though. As I get it, cached memory will be made available for other programs if needed.