I just rewatched this recently, and it definitely does make him look suspicious.
At this point, he believes his wife has genuinely been kidnapped, and so what motive would he have for following the treasure hunt clues without the police? What's he trying to gain by doing that?
I think he had no clue that the brown house was referring to his dad's house.He just found the next clue by luck.
He did mention that he would go check the house weekly to make sure it's not damaged,and I believe that he was telling the truth.So,when he discovered the letter just a few seconds before the police busted in,he chose to hide it from them to prevent himself from being suspicious.
It's just my thought:P
He did know that the brown house was his dad's house. He told Margo the story about how it was part of a make believe story he would tell himself about their dad when he was a kid. Previously that was something he had only confided to Amy.
shareThe way I recall it, he found the first clue by coincidence. But by the second one, he began to be suspicious about if it actually wasn't Amy, trying to frame him for an eventual murder. He sort of began to understand the "rules of the game".
So what he did was to follow those clues, to be the one who first found the next and destroy it, before the police found it. So he wouldn't be the prime suspect in the investigation.
The previous year the clues led to dying roses in the yard. He had no clue what she would do to one up that one but it was sure to cast him in a bad light - which would bolster the opinion against him. He knew very well about the "Brown" house.
shareI think it's because of the red panties they had found in his office- he realized that Amy knew about the affair and didn't want the police to find out. He also knew that Amy would have been livid about it and make this clear in the treasure hunt. He simply couldn't take the risk as he knew what amy was capable of (or at least thought he knew), especially with the police watching.
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