A hidden entrance in Core City does not reveal itself without having 11 Perception, even when you have the Snooping feat supposedly pushing you to 13 Perception for this very purpose. This seems to suggest that Snooping won't always work as intended, which is kind of problematic since that is literally its only purpose.
I assume that hidden entities need to be flagged manually to be "Snoopable", which would explain this issue. Either that or something else is going wrong under the hood. The way the Snooping code works probably needs to be looked at again to prevent this sort of thing from being able to happen in the first place. If one of these hidden entities isn't working properly, then there's bound to be more.
I've included below a link to a save file at the relevant location, with a level up ready to be used on the character. You can see how increasing Perception makes the hidden entrance visible, while increasing Snooping's effectiveness does not:
https://www.dropbox.com/s/0oacvnlgevcqptd/Bugtest.zip?dl=0