I've used the dye and the electronic sensor. Both with mixed and sometimes poor results. I've pulled a vacuum on some AC systems and they'd hold steady for 24 hours. Then I'd charge and it would leak out in a week. Sometimes a leak will not show under vacuum or under high pressure. Some leak when shut-off at low pressure.
I've had the most success with the problem mystery leakers by hooking an air-chuck to the AC and pressurizing with 20-30 PSI. Then spraying with a hand-held spray bottle with soapy water. THAT has found problem leaks for me when nothing else would.