Every year we get quite a few, up to 10 or so, they are signed with a first name and there's no return address on the envelope. Sometimes we don't know who it's from because we know lots of people with that name, and sometimes we just don't recognise the name. It's easier if it's from a couple, but even then we usually get at least one that we don't know who the couple are.
Our very first card this year arrived today and it's an unknown (from Graham, I don't know any Grahams and DH knows a fair few).