Blank_User said:
If you have the post ID, you should be able to access the tag edit history of that post and get the source link, though there’s no guarantee the link will work.
It's very easy to get any post ID with post-notice search. For example, coolio favourited post #1521834 (Explicit) (though, it works with literally any post, it works with post #1), now we add "?q=loli+fav%3Acoolio" (generated by search of loli fav:coolio for post-notice search) to the link (to make it look like danbooru.donmai.us/posts/1521834?q=loli+fav%3Acoolio) and here we go, every user can see gold posts (still no images for blue users) just by pressing "next" and "prev". Blue users can see links, tags, copyrights, pixel hashes, etc. Since some sites copy danbooru posts (looking at gelbooru with their danbooru bot) it becomes pretty easy to find almost everything. Plus, that way you can get takedown posts. For example, post #9465758 (Questionable, banned artist, but active), we add "?q=ikemeru19" to the link and go next. We get to the post that was taken down (but blue, gold and platinum users still can't even see tags), now just preview it as dtext like post #9192404 and if you just hover the post (tooltips must be enabled) there's a source link and tags (still no images). And while the interface for post-notice search disappears you can still navigate through active posts, like you can take next available post #9192402, add the same "?q=ikemeru19" to the link and you'll get post #9138226 (Q).
Updated by Gardares