The custom of such guhya or secretive Bhairava can be an oral tradition that is continuous within the guru disciple custom. Their karmakand rituals and methods are magic formula, they're not composed down but only learnt and practiced in guru disciple tradition. In the Kala Bhairav Ashtakam by Adi Shankracharya, https://rowanheyrl.webbuzzfeed.com/30386573/bhairav-an-overview