I just realized my some of my notes had disappeared. I checked my gmail, and as people have said I found a couple of notes since the Notes app had switched default account without me knowing about it. Mildly infuriating.
However, there were still some important notes missing.
This is an ugly way I found to get some of the information in the lost Notes back (in OSX):
1. Open Finder
2. Click "Go" in the top menu bar. Hold down the "option/alt" button (on your keyboard), and the option "Library" should appear in the drop down menu. Click "Library" (it's hidden by default)
3. Open Containers, then com.apple.Notes, then Data, then Library, then Notes
4. Here you should find a couple of files. As far as I can tell, this is where your Notes data is stored. Unfortunately, there are a lot of junk in here too (formatting info etc etc) (they aren't intended to be read this way), but if you really need the info of your lost notes it might be worth the treasure hunt.
5. Open them in textedit (right-click, open with, other, then choose textedit or your text editor of choice). Don't save any changes you make to these files!!! Actually, it might be best to "save as" to a different location just to be sure.
6. I found the most valuable info pretty close to the bottom of the "NotesV1.storedata-wal"-file. Don't forget you can hit Command+F to search.
Hope that helps!
Ps. It is insanely embarrasing that Apple can't figure out how to get a basic Notes app to work in a secure way. :-P