A few years ago, I got called to see my boss. Apparently, the switchboard had notified him that I had made a *lot* of calls to india, which were very expensive. Can't remember the exact details, but there where hundreds of pounds worth of calls made from my desk phone.
I was called to explain this. I explained I did not make personal calls on my office phone (I have a mobile for this), and that I did not know anyone in India. While I did have international calls enabled on my desk phone (by default, international calls were disabled on the company phones), this was purely so I could deal with some of our US based suppliers. I did not call any other country, and only called the suppliers on rare occasions. He didn't really believe me so I asked if he could give me a print out of my previous calls, so I could look into it.
He did, but had to request it from the Switchboard, so it took a couple of days.
When I got the print out, I spent about 5 minutes going through it before I noticed something odd. All of the non US international calls were made when the building was closed. While I often used to work late, even if I'd have wanted to, I wouldn't have been allowed in the building when it was closed. The only people allowed in were the security and cleaning staff, so I highlighted all the calls made while the building was closed, and took the printout back to my boss. He apologised, and said he would deal with it.
Never heard any more, but I did notice that a security guard I had considered a bit of a friend vanished after that date, so I suspect it was him.