From the Super Mario Wiki, the Mario encyclopedia
In the Nintendo 3DS version of the game, the British Museum serves a much larger role, appearing as the location for two of the Story Mode episodes. The museum is mentioned by Rouge in The Mysterious Thief, where she claims she was visiting the museum when Dr. Eggman arrived with a Fog machine and brainwashed her, and also by Jet in Enter Bowser?!, who points out that there is Phantasmal Fog there and suggests that Sonic and Tails should investigate it. Mario, Luigi, Toad, Sonic and Tails first arrive outside the British Museum in Mario and Sonic, where the group finds the purple Phantasmal Fog and share their findings, before defeating a group of Fog Imposters. Mario and Sonic then open the gates of the museum and the group head into the courtyard. In The Secret of the Fog, the group search the courtyard for a Fog machine and find Eggman Nega, who leaves after being defeated. Mario and Sonic find and destroy the fog machine, clearing the fog from the Museum, and head inside, where Tails finds out that Dr. Eggman had visited its library and borrowed a book containing information about the Fog urn, which had been recently stolen from the museum. The group area approached by Esipo and Charmy, who ask for their help defeating Bowser and Dr. Eggman's minions who are causing trouble around the city, and then split up before leaving the museum.