MM, the “ghost listings” you are seeing are likely “Exclusive Agency” aka “pocket listings” which are pretty common in older areas where if something comes on the market, it is usually one-of-a-kind and gets pounced on right away. For example, it is common for probate listings to be “pocket listings.” Many agents keep lists of prospective buyers who have been waiting for months … or even years to buy something in a particular micro-area (the agents “farm”) or even a particular street. Often, there is no need for an agent who gets a listing in their longtime “farm” to market it on the MLS right away and offer a co-broke fee right out of the gate. Their seller can always consider any offer coming from a buyer represented by another agent and decide on a case-by-case basis whether they are willing to pay another 1-2% commission than they agreed to (to give the buyer’s agent at least 2.5% of the commission) based upon the terms of the offer and accept, reject or counter the offer.
Hence, you won’t typically find these listings on the major online MLS aggregators.
IIRC, zillow even shows photos and details of properties which haven’t been listed or marketed in any way for many years.