Michael seems to feel that he should test every AC unit for “possible” leaks.
You are correct in that if possibly you suspect a leak,THEN and only then, a detector can confirm and pin point the leak. I am of the opinion, if I find an indication of a leak, or the system is not working to full capacity, I refer it to a AC repairman for further investigation. We are NOT mechanical system repairmen, nor is it our job to isolate the problem.
All gadgets should be used, and only used to help you confirm what you suspect, so you are correct in that matter.
My opinion is if you are not a qualified repairman, refer it.