Late, late, late, but I feel I should point out that there actually is a way that Dhaos could be identified, or at least put in the path of very, very deep suspicion. There's a spell in the Buffyverse, translated name "Part the Veil", and its purpose is to expose the presence of magic or spellwork.

It exposed Dawn as a fabrication, and up until that point everyone believed unequivocally that she'd been with them for years when she'd really been created last week. Not only that, but the spell manifests itself in a way that is meant to explain to the caster just what is going on - Buffy, for example, looked at family photos and saw Dawn flickering in and out, looked in her room and saw it empty, all that stuff. It's also a spell Buffy has cast successfully, so Giles would feel safe relying on her to use it.

Also, Helios and Ginia are horrible about keeping their mouths shut. I'm pretty sure someone will point out the reflection to him.

I'm not saying Dhaos is definitely going to be outed, because even if he is it'll take some work, but he's definitely up against some cross-canon competiton here.