Stock standard reply for when you can't find the leak.
A/c and refrigeration systems don't and can't leak. If they do they need repair.
Always amazes me when people get their fridge or car "topped up" every year.
If it happens again, you should make sure they find and fix it, especially during your warranty period. You can be absolutely sure they will find it when you're paying for it
- also it is
illegal for them to 'top up' as there are strict laws on HFC/HCFC leaking into the atmosphere - these guys know the penalties.