This lessons specifically states that:
To conjugate apparaître in Le Passé Composé (Indicatif), both auxiliaries avoir and être are perfectly valid and interchangeable while the meaning remains the same. In terms of usage, être is used more often than avoir in colloquial speech.
I've seen the comments below about one is used more for appearing, but why is mine wrong?
Soudain, j'ai apperu derrière eux
