Intent to sell, possession and paraphernalia

My boyfriend was recently arrested Friday night. He is being charged with Marijuana possession, paraphernalia, and intent to sell in the state of Maryland. However, the story goes (and I know it to be true as I was there...) that him and I had gotten back from a concert that night, and as we did, his roommates had decided to throw a party. We walk back into his bedroom and there were tons of people, smoking marijuana. Right as we get them to leave, the cops arrived from a noise complaint. They smelt marijuana and claimed to have seen some on the table and entered the house. They asked where it was being smoked, which lead them to the back room (where everyone had also ran, since it was the furthest away). They then asked where it was, and my boyfriend showed the police the very small amount he had in his nightstand. THey then allowed everyone to leave after IDing them, and then searched the house. In my boyfriends room, people had ditched their backpacks with marijuana in it. There were a good 5 or 6 backpacks, which a few had scales, baggies and marijuana. Inside one bag there was a notebook containing numbers and prices and all sorts of selling information. Even though there was a name on the notebook, they claim it to be my boyfriends. Since all this was in his bedroom, he is being charged for everyone's marijuana and paraphernalia. I would just like to know if this is possible, as the police clearly knew there was a party (they entered with lots of people there/arrived for a noise complaint), but allowed everyone to leave before they searched the house. I can't imagine him being charged for something he did not know people had left in those backpacks, let alone his room. Please give me some kind of advice/help!