This is probably a Realty Trac listing. This company takes Notices of Default and foreclosure and posts them information on various sites, including Trulia. They make their money by having people subscribe to their service. The buy their data from sources such as Dataquick.
The subscribers approach the owners that are in difficulty and offer to buy their home.
So it is not really a "listing" in the typical sense. That's why the agent did not find it in the MLS