Heh, the thing is I was IN cosplay at the time, so I am not sure it counts, but the most random place I've spotted cosplayers is on the Skytrain in Vancouver, Candada. It was when Anime Evolution was still running so all we really did was look across from one another until I had the guts to ask, "Anime Evolution?" they said, "yes," and that was the extent of our exchange.
I have to admit I usually am the person you'd comment about seeing in a strange place in cosplay. I used to stay with my mom for Anime Evolution so I had to take public transit every single time... for about eight years straight that I went I got strange looks heading to that convention.
"It's amazing what a performer will do when the camera is rolling... it's probably a psychiatric illness!"