Answer: Your manager is not only worried about a sudden resignation, but also may feel a bit betrayed. To repair the damage, go back to him with an honest, but politically intelligent, statement of your intentions.
For example: "Like many people, I occasionally get curious about other opportunities, so I did explore a couple of positions. However, I have no plans to leave. I like my job, and I don't want you to think that I'm about to bolt out the door."
If you’re a valued contributor, that should secure your position. Your boss is unlikely to ditch a seasoned employee for an untrained newbie.
This experience clearly demonstrates the wisdom of keeping job search information to yourself. If you tell anyone, word can easily spread to your boss. And as you’ve learned, that can lead to career suicide.
For Office Coach tips on looking for work while you’re employed, see Job Hunting When You Have a Job.