Thanks guys, very helpful.
As I mentioned already, I updated Evernote. From within Evernote itself, not from a DMG. This lead to the application being moved to the trash, and a new one took its place instead. From a user perspective, this is seamless and you don't realise that it is actually installing a new application instead of replacing the existing one. So, there's still hope to recover the old notes which Evernote for whatever reason never ended up synchronising with the web server.
So no, nothing to do with the user who is 60cm away from the computer screen who is also, by the way, a software engineer.
Most likely someone will end up in the same situation as me, and I was able to solve it.
Thanks so much for your valuable input.