Nothing to it...
On our highly decentralized company with about 40 different office locations, I started my job there assisting with a large project to replace CRT screens for LCD ones, inkjets for laser-printers and replacing ~100 XP machines with thin clients. Ok, so I was new.
The first thing you do of course when you enter an unfamiliar building looking for a department where you don't know anyone is applying basic courtesy and introduce yourself. But after having met the so-many'th uninterested office worker, and such a long line of offices still on the todo list, I started to play a little with it.
I went into an unfamiliar office building. Tried some doors, until I found one that was unoccupied and the door open, and walked out with computer equipment. Crazy! But with this success, I went further. I asked random people if they could unlock locked offices. And they did, without me even introducing myself, and I mostly wasn't questioned. The few times that I was questioned, I just said I was there to take away computer equipment.
Note that none of the things I did was announced. Roughly 95% of the people didn't know me. We have no company clothing (in fact, I'm a shorts and sandals type). And just for sport I made sure I went in empty handed and walked out with equipment.
Of course, with this job being legitimate and all, I made sure I'd bring in replacement equipment, and made an effort to get introduced to the office workers. But not before I had my fun with pretending to 'steal' equipment.
In all this time, only one user made an effort to verify my actions with her manager, and one who denied me access until I was able to convince her I was legit. This as opposed to me borrowing some six or seven master keys, and having bin in dozens of offices unsupervised, and walking out with loads of equipment unquestioned.
All this trust in a fellow human is of course beautiful. But I don't think it's an inherent trust that allowed for this. Rather, disinterest and/or fear to confront someone. But it also has to do with attitude. If you walk in with a determination to do something, people recognize that and will comply with you quite easily.
One last example, as it stands out above the others. We have a couple of small (often part time) support offices in the office buildings of other companies for some joint ventures we have. Really nobody knows me there. I walk in, and I have not a clue who to turn to or where to go.
But the human hierarchy works a bit like DNS. Nobody knows everything, but everyone knows someone who might know more. The first person I met was a cleaner, and I ask who could know where I should go. Somebody else gets called, and she knows the office I need to be, but it's locked. Long story short, the president of that company is the only one with access to the key I need.
So he gets called from the meeting he's in, and I borrow his personal keys (home, car, a dozen others) and I have the master key of his company. Now I've only given him my first name, made a vague reference to that I 'work in IT' and that I 'have to be in that office'.
The moral of the story, kids, is that you hardly have to shave, don't need a suit (sandals and shorts suffice) and you can leave buildings much richer than entering them. But remember that stealing is illegal in many countries, so be sure to consult your local authorities if you are in doubt about applicable laws.
/This on the subject of (in)security, please excuse the long post.