Jealousy

Gaius had just been kidnapped! Merlin was really angry about how little Arthur seemed to care about that! Gaius has been by his ever since he was...well ever since he was born actually. How could he just turn his back to him like that? How could he think he was a traitor?  How could he think that Gaius would leave without saying his goodbyes, if not everyone then at least to him? Arthur was a great man, but Agravaine was poisoning his soul and his heart with his words, his ideas, and his actions.

The worst part was that Merlin knew Agravaine was working with Morgana. He had seen them together. He knew who the real traitor was. But he could not say a word to Arthur, he could only look from afar. Agravaine was Arthur's uncle- his last living sane family. Merlin could not bring himself to destroy his best friend's last chance at family. Besides, if he was to say anything, he would probably end up dead anyway. So he decided to leave. He decided to go find Gaius and bring him to Camelot.

In the cover of night, Merlin left Camelot and started tracking down Gauis. He tried to leave all the dreadful thoughts behind him. He tried not to think of all the reason why Morgana had wanted to kidnap him. He tried not to think about the tortures that his adoptive father was suffering. Merlin used his magic to find the place where the court physician was held captive. Saving him was his priority. Saving him was the only thing he could think of.

Which is probably why he didn't see or hear the knight following him.

Without any hesitation, Merlin entered the cave. At one point he had to choose between saving Gauis and getting Morgana out of the picture. He had chosen the latter.

Gwaine knew that Merlin wouldn't leave it be. So he had been waiting at the entrance of the forest since Arthur's sentence had fallen. He was waiting for Merlin, because he was sure that the young servant would never ever accept that Gaius was guilty of treason. And he was right; he had just seen a shadow enter the forest. So, out of concern for his friend, he started following him.

Merlin was riding fast, without hesitation as if he already knew where Gaius was held captive. When the knight saw Merlin entering a cave without even looking back, he followed him once again.