Your potential employer does not want to hear that you are finding out about their job by accident.
They don't really care that you like them; they want to know why they will like you.
You do not "drop by" an office to see if they will tell you more about the position.
Please visit a few job posting sites and review their sample cover letters. I'm sorry to say that yours would have been thrown in the recycle bin the moment I saw you found us by accident.