You don't consider visible signs of leaks a reason to remove them from service? As far as I know, the leak could have just started a few hours before I turned up to do the service - you have no way of knowing if it'll be empty in a few days. This is not something you can just say, "oh well, as of today, it hasn't lost more than 10% of the contents so I'll let it go until next year". A visible leak is a defect in a piece of potentially life saving equipment and I want to sleep at night thank you.
Any TG cans we find on our service visits are removed from service and replaced under manufacturer's warranty at no cost to the customer.
Any leaking extinguishers spotted at a fire risk assessment are brought to the attention of the site staff so they can arrange their service company to swap them over.