John 18:16

But Peter was standing at the door without. So the other disciple who was known unto the high priest, went out and spake unto her that kept the door, and brought in Peter.

The circumstance of John's being favorably known to the high priest was that which provided this eyewitness account and also resulted in Peter's admittance to the theater of his triple denial. Thus the question left dangling in the synoptics as to how Peter happened to be at the trial, or near it, is cleared up by this circumstance, as is also the status of the "damsel" who questioned Peter.