Oh I feel your pain on this one. I've actually had PM's dig through my files and send unfinished work straight to our developers because they didn't realize that it was discovery work.

I've found a couple little tricks useful for this. First, I use pages to separate rough work from anything ready to review. I use an "" to denote that a page contains unfinished work, so "Discovery" is a page that shouldn't be reviewed, where "Prototypes" are ready to review. I also include a cover page for prototypes and recently added a callout to it that indicates the status of the prototype so that even if you don't see the pages, you get an idea of where the work is at. Lately I've just been keeping rough work in my drafts and waiting until it feels pretty mature to move it into one of our team projects.