Isn't some kind of signage at least required?

You're right Roger, it's very scary
When I was 12 or so my friends and I went into an abandoned generating station. I can tell you that we had absolutelyno clue as to what was in there. We saw the whole thing as a 20 story jungle-gym.
After the cops caught us (I guess we gave it away when we started the World War II air raid sirens), we were told that there were still lots of HV lines that were still energized.
I'm very thankful none of us were killed.