Freon is now a finite resource because it is no longer made by any
civilized country (may still be made in countries such as China) since it
is proven to destroy the ozone layer.
So an a/c system should never be 'topped-off' before being tested for
leaks. If there are any leaks, they should be fixed before Freon is added.
Any competent a/c mechanic should know this and act accordingly.