Dye is heavier than water. When it is released slowly beside a suspected crack or fitting, with the pump off and the water still, it is drawn into an opening that is leaking and drifts away from one that is not. Dye confirms a specific spot; it cannot search the whole pool.
You can try it yourself at a skimmer throat, a return fitting or a visible crack using a syringe and food coloring. Keep the pump off, wait for the water to go still, and hold the tip within an inch of the spot. If the dye is drawn in, that spot is likely leaking. If it is not, you have ruled out only that spot.
Still not sure it’s a leak?
Describe what your pool is doing, and we will tell you what it may be and what a visit would involve. If the water level is dropping fast, call us right away.