It's not really hidden once the billowing white smoke comes out the tailpipe.
The coolant dye test is for locating coolant loss when you do not see smoke or you do not see the obvious dripping or pouring leaks like hoses, radiator tanks/seams and/or water pump weep holes.
The dye comes in a 2 or 3 oz bottle so no draining of the system is needed unless the customer has added a stop leak product. The Radiator only pressurizes to 12 to 16 PSI while the combustion chamber sees 120-160 PSI just cranking.
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Usually a exhaust gas sniffer is used on the radiator with the cap off to sniff for exhaust gasses in the coolant if you don't see obvious bubbles from the combustion pressure going into the cooling system.
Regards,
Doctor Olds
