Cake is Not a Lie: Edit a post which goes on to earn 5 upvotes.
A cupcake that goes on your head http://cdn.prom.sstatic.net/WinterBash/img/1900909705-128.png
The Abby: Participate in closing a post.
A red bowtie http://cdn.prom.sstatic.net/WinterBash/img/1432640864-128.png
N00b hat: Downvote a post.
A multicolored propeller beanie http://cdn.prom.sstatic....
Winter Bash ran until the 5th, and then, as the site, they were put back in their boxes. Not sure what sort of notice you expected, but they were meant to be a limited time thing, and thus are no more.
You can't know! If you do manage to work it out though, there could be more secret hats in it for you...
All secret hat secrets will become unsecret after winterbash.
Some secret hats have been desecreted here too.
Leaderboards are heavily cached - they'll update eventually. The correct hat count is visible on your profile page (which will show your overall hat count across all sites, rather than the individual hat count for the site you're on).
If you want your current per site hat count, the leaderboards are your only option (without manually clicking on each hat to ...