While I’m sure a homeowner could in fact manage to trash their house without leaving any real evidence as to who did it, I can think of any number of ways that MOST homeowners who trash their houses could be caught.
Did they snag everything even remotely portable to sell on Craigslist/E-bay? Trace that sale/auction posting back. Something tells me that sink came from somewhere.
Do they have neighbors? Neighbors can be excellent witnesses – especially since I suspect that anyone who WOULD trash a home they no longer own may not exactly be the best neighbor – it suggests they have anger issues.
In any case, the banks aren’t pursuing them – most likely because their legal departments are already swamped doing all the actual foreclosure work. It’s simply time and money they’d rather spend on other issues at this point.