I think I've seen two, both on long perms with the same clubmate.
One was about 2am, on the Twilight 600, on the long climb out of Dalmally. My clubmate was way ahead of me, but I became aware of a cyclist just up the road a little. I thought it was strange that he was going so slowly, but as I drew nearer, I realised it wasn't my clubmate at all, but an old guy on an upright roadster. "Fine night" he remarked as I went past. I was mighty relieved when I eventually made contact with my clubmate, and he confirmed that he had seen him too.
The other was a few weeks ago, approaching Glendevon, again in the dark. We were together this time, and noticed a cyclist ahead. He seemed to be on a touring bike, panniers etc, but was wearing a huge flapping tweed greatcoat. "Fine night" he remarked as we went past.
I sometimes wonder if there's a Flying Dutchman thing going on -- if they breathe a sigh of relief when they get a puncture, free at last to take a ten-minute break before riding round Dartmoor one more time...
Maybe there's a Flying Scotsman thing going on as well.