So I'm (spoiler clearing nests out of the Forgotten Tomb), and I have one more to go. Then I find an abandoned Sylesti egg. I get the usual question of do I want to adopt it or leave it. I adopt it.

Voila! Suddenly, the Quest is complete. (Evidently, adopting an egg counts as "clearing a nest.") Has this happened to anyone else?

Nope~! Never been lucky enough to find an egg while doing that quest. :O I'm lazy and have done it maybe a handful of times only though, so no surprise there. If anything, it's good information to keep in mind? xD Grats on the egg.