Go to the second floor of ONBS. Inside of Secc's office, he notices your Snag Machine isn't there. You explain to him what happened. Secc tells you about Team Snagem. He then asks you to see a man named Hordel in Outskirt Stand. He marks the place on your map. Make that your next destination.
Your Miror Radar will go off and you'll find out Miror B. is at Outskirt Stand. As you try to enter the locomotive, Folly and Trudly will come out. Miror B. follows suit, sees you, and battles you.
You still can't Snag Nosepass. You'll have to knock it out, like you did with Zangoose. Use the same strategy you did when you beat him before. He does have a Ludicolo now, but it still shouldn't be too hard. After the battle, he escapes.
Enter the train. You'll see the ONBS news about the whereabouts of the S.S. Libra. Then talk to the Scientist near the entrance of the stand. This is Hordel. He used to work for Cipher when he heard what they were doing to Shadow Pokémon at the Cipher Key Lair. He says he managed to get away with a Shadow Togepi. Talk to him again and he'll ask you to purify it (make sure you have an empty spot in your party).
Use the Healing Machine on your right. Go north to get the item chest containing three Ultra Balls. Go left to the stairs. Go down. Behind the vending machine are two Hyper Potions. The Snagem goon nearby will battle you.
Near him is an item chest containing a Rare Candy. Go up, past the other Team Snagem member that you beat, and through the gap in the wall to your left. Go down, just below the stairs. You'll meet yet another Snagem goon.
After defeating him, Gonzap will give your Snag Machine back. He sees that you and him aren't really different, and you both want to bring Cipher down. And he tells you the Snag Machine didn't fit his massive muscled arm anyways. Then, he'll give you Gonzap's Key. In the bottom right corner of Gonzap's office are two Full Heals. Remember that item chest that was locked? Go back to it and use Gonzap's Key on it. It contains TM30 (Shadow Ball). Pretty useful if you ask me.
When all is said and done here, remember how you missed Miror B.'s Nosepass? Let's go find him.
Keep running into different towns. Eventually, your Miror Radar will go off. Depending on where you find Miror B., he'll have a different team, but the Shadow Nosepass will be there. All of his teams are a random amount of Pokémon chosen randomly from the ones below, plus Nosepass. In a Colosseum battle, Miror B. will randomly replace one of the four Trainers in the round.