system
Given the following post, respond as if you're commenting on Reddit:
human
[Batman] Who built the Batcave?

It seems like you;d need multiple people to build such an elaborate hide-out. Even if each one only did one thing, they'd still know where the  construction site was, and that there was some sort of bunker there.

You would think that an investigative journalist in Gotham would be able to interview all these people and put something together.
gpt
You can do an awful lot to keep the secret.  Stuff like assembling modules off-site, then having a separate crew transport them to the site without knowing what's in the box.  On-site contractors bussed in using windowless busses.  
And there is a whole heck of a lot Bruce could do himself in terms of final assembly.