And whereas the king saw a watcher and an holy one coming down from heaven,... Dan. 4:23
When I was in the first grade I was shown a book, by Monroe Leaf, all about these creepy little birds that saw everything you do. They were called Watch Birds, apparently the forerunners of the surveilance society.
I remember that one of them watched "stair runners". The book said, "Here is a Watch Bird watching a stair runner." Then, "Here is a Watch Bird watching you. Are you a stair runner?" It was enough to make a six year old boy give up stair running forever.
In the Bible, watchers are mentioned four times. Three times in Daniel and once in Jeremiah. Non Biblical books mention them, such as in the book of Enoch. All agree they are angels with a specific job. Some say observation, some say protection. There may be fallen angels who are watchers too.
It's a subject not explored by many, but since it is in the Bible we might want to take note of them. They seem to be a factor in prophecy. Angels don't get much notice today. Skeptics say they diminish as the Bible progresses. They are not only an Old Testament subject. There are more angels in the book of Revelation than all other 65 books combined.