I'm probably too late to the party. But, just in case, I've got a non-ghost story, but still managed to creep me out a bit. A few months ago, I was working from home on a Friday so I could finish up a presentation for the owners of my company without the distractions of the office. I had my remote desktop session open to my work server, Netflix, Steam (pre-downloading Bioshock Infinite I think), and probably some other random stuff all running on my computer. I had been working on a report for several hours and was watching Futurama on netflix as I finished up editing/formatting some of the excel files. Once I was happy with it, I saved/closed it. I hadn't quite closed out my Remote session when I noticed that there was more sound coming from my speakers than just Futurama. I assumed it was an ad on a webpage that auto-refreshed or something (I had a ton of bookmarks opened). So, I alt-tabbed to chrome cycled through all my open tabs. Nothing. There was a guy with an English-sounding accent talking. I paused Netflix and could hear it more clearly. It was clearly a guy chatting and playing a shooter. I could hear explosions and gunshots in the background. I could hear the one English accent guy talking to someone or maybe a few others. He was describing the game, like he was introducing friends to it.
Figuring that it was some glitch, I sat there listening to it for a minute. I pulled out my phone and tried to record video of it. Of course, my phone was being a POS and it would only record for 1 second and crash. So I have 2 1 second vids of it. After that, I became more interested in HOW this could happen. So, I took closed every running program on my computer, one at a time. I closed Remote Desktop, still there. Then, Netflix, Chrome, Excel, Word, logged out of steam. And it's still here.
At this point, it dawns on me "maybe this could be 2-way?". I pickup my headphones and ask in the mic "hello? Can you hear me?". The English accent guy says: "yeah mate, we hear you? Who is this? Please keep this channel clear." I ask "what game are you playing? I didn't join this chat room and your voices just started coming out of my speakers!" He says that they are playing Modern Warfare 3 and asks how I joined their private chat. I tell him that I didn't do anything and explain that I was just sitting here and suddenly could hear them in my speakers.
I'm not logged into steam, all running apps on my desktop are closed. I figured I'd go into Task Manager and start shutting things down there. When I ended the task for Steam, it stopped. BINGO, steam error. I sent them a support ticket and got the default "delete everything but steam apps folder, roboot, and try to re-produce the error".
Re-reading it just now, it's not that creepy now. But, at the time, it sort of freaked me out that suddenly I was connected to a random online chat. It felt pretty invasive.