Don't know his dad is Hawkmoth (okay, fine). Apparently didn't discover the book was about the powers of the miraculouses. He doesn't know about the guardian and simply accept when another miraculous holder appear. Add that Plagg doesn't make an effort to explain things, because he doesn't remember or simply doesn't care anymore. Also, his mother could possibly have being a miraculous holder.
I know that a basic recipe to becomes an anti hero/villain it's be locked out of the important things, but given his initial reaction to Gabriel "supposedly" be Hawkmoth on season 2 premiere, how would Adrien react to many other secrets? He would learn searching by himself? Or does he simply accept when his partner appears with a solution from Fu's miraculous' casket?