I think this is the biggest risk when it comes to a possible new outbreak. Some uneducated people clean out a lab of storage facility and just throws everything in a dumpster without knowing what they are working with.
It has happened before with other stuff (medical records, computers etc.) and it will happen again. The question is if there is something somewhere that is a major danger. Even worse is if there are some vials with biological warfare material that makes Ebola seem like a common cold. Since much of that research is done secretly it's not easy to know - and in some cases everyone that knows may have passed away and the remains of those projects are just stored in a warehouse with a reference to some documents that have been shredded a decade or more ago.