23 But it was not when he was outside at the gate that he denied the Lord the second time. That took place after he had come back to the fire-place. There was no need, however, to mention the precise time at which he did thus return. Consequently Mark goes on with his narrative of the incident in these terms: “And he went out into the porch, and the cock crew. And a maid saw him again, and began to say to them that stood by, This is one of them. And he denied it again.” This is not the same maid, however, as the former one, but another, as Matthew tells us.
Nay, we gather further that on the occasion of the second denial he was addressed by two parties, namely, by the maid who is mentioned by Matthew and Mark, and also by another person who is noticed by Luke. For Luke's account runs in this style: And Peter followed afar off. And when they had kindled a fire in the midst of the hall, and were sat down together, Peter sat down among them. But a certain maid beheld him as he sat by the fire, and earnestly looked upon him, and said, This man was also with him.
And he denied Him, saying, Woman, I know Him not. And after a little while, another saw him, and said, “You are also of them.” Now the clause, “And after a little while,” which Luke introduces, covers the period during which [we may suppose that] Peter went out and the first cock crew. By this time, however, he had come in again; and thus we can understand the consistency of John's narrative, which informs us that he denied the Lord the second time as he stood by the fire. For in his version of Peter's first denial, John not only says nothing about the first crowing of the cock (which holds good of the other evangelists, too, with the exception of Mark), but also leaves unnoticed the fact that it was as he sat by the fire that the maid recognised him.
For all that John says there is this, “Then says the damsel that kept the door unto Peter, Are you not also one of this man's disciples? He says, I am not.” Then he brings in the statement which he deemed it right to make on the subject of what took place with Jesus in that same house. His record of this is to the following effect: “And the servants and officers stood there, who had made a fire of coals, for it was cold. And they warmed themselves; and Peter stood with them, and warmed himself.” Here, therefore, we may suppose Peter to have gone out, and by this time to have come in again. For at first he was sitting by the fire; and after a space, as we gather, he had returned, and commenced to stand [by the hearth].
Source: The Harmony of the Gospels (New Advent)