Around 1999-2000, I used to work for a large newspaper. At the time, the department I worked for was an open office/cubicle-type affair in the original building (nearly 100 years old at the time of this story. It has since been pulled down).
I was standing at one end of the room waiting to make myself a coffee. Because of the way the department was laid out, there was no way anyone could come up behind me without me seeing or hearing them. The department was also only accessible via a key-card locked door. So people couldn't come in off the street, etc.
As I was standing there, a middle aged male voice said quite clearly in my left ear: "What's up, XXXX?" (where 'XXXX' is my name). It felt just like someone was standing right there - when someone is next to you, you can feel them there without actually seeing them. I could feel this...entity?...there. I could even feel breath on my ear as if someone was speaking very close to my ear.
I turned around to answer but no one was there. There was no one close by and no one was approaching or leaving. There was no way anyone could have left without me seeing them. I didn't recognise the voice. At the time, we would have had two men working in the department would would have fit the rough age I'd thought the voice was. Both were out of the office at the time.
I've always been slightly interested to know who or what it was who spoke to me. I don't believe I was having some sort of lucid dream or whatever - I was awake and alert at the time. Just waiting to make myself some coffee! No one had died recently (in fact, only one male relative had died in my lifetime at that stage and it had happened many years before... And he didn't sound like this guy). There was no history of hauntings that I could find in the building I was in and I was never troubled by visits while living in NZ. (We've since had a few things happen since we moved countries but that's another story!) This was the only time he spoke to me and he continues to remain a mystery!