Pre-probate is a term that is relatively new. It basically means that a property owner has recently passed but the property hasn't been transferred to another party.
Some pre-probate properties will eventually end up in probate court. But the balance of them will be transferred to the heirs via trusts, joint tenancies, community property transfers and several other methods of non-probate transfers of property.
Like I said, pre-probate properties are a relatively new niche market and as a result there really isn't much statistical information out there. My best estimation is that roughly half of the pre-probate properties won't end up in probate court. But that is not scientific by any means.
Investors, agents, estate liquidators and other professions don't have to solely rely on regular probate filings to identify deceased owned properties. Pre-probate properties reflect a market that is fairly hidden as many of these properties won't end up in probate court.
What I can provide: