I agree that you might contact the police, but I'm not sure what they can do. The vagrant could say that they are 'house sitting' for the new owner, and who's to say otherwise if they can't reach the actual owner?
If you contact your previous lender, and ask to speak to the asset preservation department, or the REO department, you might be able to reach someone who is responsible for the property.They would be my personal choice in trying to let someone know of the situation. The property is most likely in the process of being transferred to an asset manager, and then assigned to a realtor for going up for sale. The asset manager will direct either a service, or the realtor to secure the property, and eventually put it back on the market.
It sounds like it's in transition at this time but I'm positive it won't be for long. The cities/counties are charging hefty fees for properties that are not secured, and maintained to a minimum standard (lawns mowed, etc) in terms of fines. So banks are quick to set up those services once they obtain ownership to foreclosed homes.