Missing Legos(Bugatti Chiron)

A couple months ago, my brother got this lego set, Bugatti Chiron. He was very exited about this lego set. He already had the Porsche 911 GT3 RS lego. After we came back to Korea, he started making it right away. On Tuesday, my mom picked me up from my academy. I got in the car and noticed that my brother (Aiden) was very grumpy and sensitive. I thought something was wrong, but didn't ask. Then, mom and Aiden started talking about something. I asked what was going on, and mom explained everything. Basically, Aiden was furious because a bag of legos were gone. He said he put it in the middle of his room, but just disappeared. We thought that he might have lost it because his room was very dirty, but he thought someone stole it. We all didn't know where it went, and Aiden was thinking that either: a robber came, or our babysitter stole it. The first choice was definitely not it because, why not steal the whole thing? If I was the robber, I would steal the whole thing and get some more valuable things from our house like our phones. This bag of legos were the only one that was missing. It couldn't have disappeared. Nobody in our family would have stole it because we can't really go in his room; it's dirty. Also, we can't steal it because Aiden doesn't let us even touch his stuff or use it. He is very sensitive about this part, so we've had consequences. The second choice was understandable except the stealing part. Our babysitter also knows that Aiden is very sensitive about it. But after he got off at PEAI, we called her anyway. She said that she doesn't even go in his room after he screamed at her for touching his legos. She only went in there to charge her phone. Even after Aiden came back home from PEAI, he still thought that our babysitter took it. But again, it doesn't make any sense. Then, Aiden took a shower. Mom and I was thinking that this is impossible. Mom thought that he put it somewhere he doesn't remember, and is thinking about the wrong space. After he came out of the shower, I took a shower. I came out, and Aiden solved the mystery. He was supposed to make number 9 bag, and he thought he was supposed to make number 8! We think that now, he should clean his room more because this incident happened because of mistakes(dirtiness).