"I walked away without looking back, feeling like we'd just gotten away with an elaborate bank robbery."

I used to run treasure hunts, along with a friend of mine, where teams of four would solve puzzles and clues that would take them through a series of locations similar to the one in the article [1]. This is a really good way to describe the feeling -- I've always described it as "playing secret agent."

Incidentally though, we were always terrified to bring even small groups of people into these various abandoned buildings. I still have nightmares about it. Having a party -- with that many people and that much noise -- is absolutely insane.

[1] Some examples I dug up: http://imgur.com/a/WVw4Q