I've read that "up to 30%" of properties bought/sold are "off market".
My sister bought such an apartment in Dublin early this year, for about 40% less than what it cost the seller five years ago. He didn't what people to know that he was selling it.
A friend of mine bought his home about six years ago. It too was "off market". He'd mentioned in casual conversation to someone "in the know" that he was house hunting. Said person said, "you know who's got a house that he's thinking of selling... I could put you in contact".
It's a bit like the jobs market. There are a lot more vacancies out there than get advertised. Many get filled (sold) before they're commonly known about.
BTW, you've probably seen a different post on this, but a lot of houses that are "on the market" are not for sale.