The dye they puy in likely requires a blacklight to see. Stop back and let them take a look.
It’s good that you’re monitoring your coolant. Too few people do. In my experience 1/4" in 8 months is not normal, but also not enough to cause serious concern. What I think I’d do is pressure test the system. If I found nothing, I’d be inclined to carefully examine every visable element of the system with a good worklight. If I was still unsuccessful, I’d just continue to monitor it. That’s about all anyone can do.
Hey, perhaps with the UV lamp they’ll see something.