Who dunnit?

Explanation

It wasn't the gardener. He only accused the maid because of something in the past. Something made the gardener hate the maid. It was clearly stated, "Yuck, I never really liked that girl". The accusation was driven because of hate, although it was true that the maid looked nervous and wary-there is a reason behind that. The gardener wasn't lying when he said that nobody knew what exactly the treasure was except for the Doyles. He's been working there his whole life, he's loyalty can't be compared to anyone else. He had clues of what the treasure could be but he was wrong wasn't he? Aaron Doyle said that they were documents of some sort. It couldn't be the gardener because a thief wouldn't have to wait that long to steal a treasure he doesn't even know of.

It wasn't the maid either. Although she was extremely nervous, it's wasn't because she stole the treasure, it was because she and Aaron did something. Or to be precise, Aaron did something TO her. The maid said, "Ugh...You don't understand...You really don't understand. No one does, no one ever will. Sir Aaron he-", he sexually abused me, was what she was about to say. She was too young and too innocent to even try to steal it. Aaron showed her the vault so he can get a hold of her. Around 2:30 Aaron called her into his room. He tried doing something to her and noticed the windows were open-the reason why the maid looked nervous and wary, and the reason why it took 15 minutes for them to realize the windows were open. Also, the maid didn't lie either when she said that the gardener looked at her suspiciously and guilty. Because it was true, the gardener was suspicious of her, and the guilt? Maybe there was something deeper. Well, they both hated each other, that's why the accusations were driven. The maid didn't need to steal the treasure; she was already being paid by Aaron to do what he wants to her. Aaron did say, "My, she wouldn't even touch anything. Trust me". The maid is too innocent to think of stealing.

This leaves us to the last two suspects. Is it Aaron or is it the thief? Obviously, it was Aaron! The thief couldn't have done it. Why? He didn't lie when he said that he leaves a note every time he steals. It wasn't him because Aaron said, "It was a clean inside job, no evidence was left". It also seemed that the Phantom Thief was just having fun during the interview. He doesn't seem to have any idea what the treasure was or if it even existed. He only played along to fool the detective. His character suggested "playful" and that means he wouldn't steal anything as boring as a document.

Thus, in conclusion, we could say that it was Aaron Doyle all along! He seemed happy to tell us that his parents were going to head home and scold him. (He must be a masochist of some sort) He stuttered a couple of times-he was nervous because he was trying to hide something. He "stole" his own treasure. Technically, the documents (must be titles or something else) still belonged to his parents. His motive wasn't money. He was too rich to want more. He just wanted something simpler, something he always wanted, and that was attention. He was deprived of attention when he was younger. It was stated that his parents were always gone and the he practically grew up with the gardener. He said, "Hah, but I don't care. My parents are coming for me!-Ah, it's not what you think it is. I wouldn't fake this whole incident just for that. That'd be too low of me". He clearly meant the other way around. He was just a spoiled rich kid who wanted the attention he couldn't get from his parents. He even tried to quench his thirst for attention using the maid.  

That's it! I hope you enjoyed the story~ (Contest results on the next chapter)