Theory:

     He had seen the housekeeper hang the key to the kitchen door on a hook outside. He put on a pair of gloves, took the key, and opened the door. He was always careful not to leave any fingerprints.
 
     A small dog was lying in the kitchen. It stirred, made a noise, and moved its tail in a friendly way.
 
     “All right, Sherry,” Horace said as he passed. All you had to do to keep dogs quiet was to call them by their right names, and show them love.
 
     The safe was in the drawing room, behind a rather poor painting. Horace wondered for a moment whether he should collect pictures instead of books. But they took up too much room. In a small house, books were better.
Explanation:
 
Horace had taken down all of the details of the Grange house before entering it, as previously stated. As a result, he was already aware of the housekeepers' plan. Grange House's housekeepers had planned to go to the movie on that particular day. He was waiting for the servants to leave for a movie at the Grange house and he noticed one of them putting the house key on a hook outside the kitchen door.
 
Horace took the key after wearing a glove to avoid leaving fingerprints. He was quite concerned about fingerprints when he opened the grange door. One of the major reasons that helped Horace so far in all his thefts is that he did not leave any fingerprints during a burglary.
 
In the kitchen, Horace noticed a small dog napping in the corner. The dog moved his tail in a pleasant manner, made a noise, and then moved his body a little more. "All right, Sherry," Horace remarked, referring to the dog's name. According to him, calling dogs by their names and showing them some affection was the key to keeping them quiet.
 
The safe of the grange was in the drawing room, hidden behind a poor painting. Looking at the painting, he thought that he should collect paintings rather than books, but then thought that it would be better to collect books in a small house because pictures required larger spaces, which indicates Horace lived in a small house.
 
Meaning of difficult words:
  
S. No.
Words
Meaning
1.
Drawing roomA living room in a large house
Reference:
National Council of Educational Research and Training (2008). Honeydew. A Question of Trust - Victor Canning (pp 20-24). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.