Furtive (adjective) – done in a quiet and secretive way to avoid being noticed.

https://www.merriam-webster.com/dictionary/furtive

“Helga Crane felt singularly apart from it all. Entering the waiting doorway, they descended through a furtive, narrow passage, into a vast subterranean room. Helga smiled, thinking that this was one of those places characterized by the righteous as a hell.”

Understanding the word ‘furtive’ tells that the ones that Helga was with when walking in, what seems to be a hallway, were walking in a quiet manner as if they didn’t want anyone to know they were there.