Nika already knew something strange was going on when she agreed to go back along with Jon and Ethan. The logical step would have been to refuse to go back until Ethan crossed the gate but that would have alerted him and she didn't trust Jon enough to not take Ethan's side.
Instead of refusing, she agreed but instead of going back she vanished into Uriel's shadow. As usual, Uriel could sense her presence lurking within his shadow but she couldn't see or hear anything that happened in the outside because she was actually inside of Uriel's museum.
Her plan was to poke her head out of Uriel's shadow from time to time waiting for the perfect opportunity to go back, but since she was bored she started to roam the museum. She saw the damaged clay figurine of the jaguar spirit, Thunderbolt Iron and all the other artifacts, but what really caught her eye was the souvenir shop.
