'Watchman' in the Bible

Now David was sitting between the two gates; and the lookout went up to the roof of the gate by the wall, and when he raised his eyes and looked, he saw a man running alone.

The lookout called down and told the king. The king said, “If he is alone, he has good news to tell.” And he came nearer and nearer.

Then the lookout saw another man running, and he called to the gatekeeper and said, “Look, another man running alone.” The king said, “He also is bringing good news.”

The lookout said, “I think the man in front runs like Ahimaaz the son of Zadok.” The king said, “He is a good man and is coming with good news.”