Augustine of Hippo — as excerpted in the Catena Aurea on Mark 14:66-72

“(ubi sup.)y This maid is not the same, but another, as Matthew says. Indeed we must also understand, that in this second denial he was addressed by two persons, that is, by the maid whom Matthew and Mark mention, and by another person, of whom Luke takes notice. It goes on: And he denied it again. Peter had now returned, for John says that he denied Him again standing at the fire; wherefore the maid said what has been mentioned above, not to him, that is, Peter, but to those who, when he went out, had remained, in such a way however that he heard it; wherefore coming back and standing again at the fire, he contradicted them, and denied their words. For it is evident, if we compare the accounts of all the Evangelists on this matter, that Peter did not the second time deny him before the porch, but within the palace at the fire, whilst Matthew and Mark who mention his having gone out are silent, for the sake of brevity, as to his return.”