The project you're thinking of, was that say fifteen years or so ago�? I remember something like it but in the project I'm trying to recall she didn't necessarily ask for secrets, it was more like "what thoughts went through your head the moment before I stopped you here in the street". The reply signs those people wrote and held up for the camera would say anything from "my feet really hurt" to� well, to secrets, I suppose.
There have been several similar projects since, for instance
The Thought Project by Danish Simon H�gsberg. Also, there's another, ongoing project called
Post Secret - it's not what you described but might still be of interest. Click the pointing hand for the latest postcards.