After the third denial, a rooster crowed. At that moment, Jesus looked straight at Peter. Not a word was said. Jesus' look said it all. Then Peter remembered what He had said, "Before the rooster crows today, you will disown me three times." Peter's heart was no doubt stricken with grief, remorse and shame, so "he went outside and wept bitterly."
Peter was easily recognizable as a one who had been with Jesus. One who walks with Him should be recognized as one who walks with Him. Followers of Christ should take on His likeness. Such was the case with Peter. Such should be the case with every believer.
And just as with Peter, the Lord's eyes are ever upon us too. He knows our every thought, word and deed. He knows when we do not honor Him. And when He calls us out, when He gives us "the look", if we are truly followers of Christ we will respond as Peter did, with grief, remorse or shame that lead to a heart of repentance.