Why did McMurdo hide letter from Ettie?

McMurdo was actually reading the letter when Ettie entered the room. He tells her that she cannot see the letter because he is under oath... because it contains information about lodge business, and that it might have previously been in the hands of a detective.

“I’ll tell you, acushla,” said he. “I’m under oath not to show it, and just as I wouldn’t break my word to you so I would keep it to those who hold
my promise. It’s the business of the lodge, and even to you it’s secret. And if I was scared when a hand fell on me, can’t you understand it when it might have been the hand of a detective?”