It's from the conversation with Linnae and Kaija that happens right after the first templar meeting where Medb is introduced and makes demands for the dissolution of the templars. Linnae asks the MC to stay at the training hall and asks Kaija to take him to neverwhere. The MC asks what neverwhere is and Linnae talks about how it's kept hidden.
MC: I still don't get how it can remain hidden. We have media, internet, all those things. Somebody would have talked.
Linnae: Of course. But you need to understand that a lot of rich and powerful persons in this world have connections to Neverwhere. And they make sure that it stays hidden. You can see people talking about Neverwhere in this world. They usually wander the streets with cardboard signs. Or they post conspiracy theories on the internet. Nobody takes them seriously. From time to time, somebody who people would take seriously tries to talk. That's when the important people of Neverwhere intervene. Usually these kinds of talkative people have "accidents."
MC: They get killed? And you don't do anything?
Linnae: We protect mankind from things it has no chance against. We do not protect it from itself.
Thank you, that's the one.
And frankly, I think Elea would absolutely be taken seriously if she tried to share it with others.
I don't think she would share it worldwide (not that kind of person), but potentially with enough people that it gets her on someone's sights? Yep.
Heck, I think that actually is what will (sort of) happen. She will find out about Neverwhere and end up in danger.
That's what whole "encourage her to keep looking or tell her to drop it" choice was about in my opinion.
EDIT: Let me re-phrase slightly. She wouldn't get it danger for sharing it with others, she would keep investigating and exploring more and more and more, until running into something that she is not supposed to see, and that puts her life in danger.
At that point, Templars would just shrug and drop it. Just another person in danger, not their responsibility.
That's why I'd prefer to tell her all about the place, to avoid accidentally stumbling in some serious trouble.