Certainly from what I'm aware of, there is far more movement of witnesses between countries, particularly within Europe, than there has been in the past. There is a specific network of witness protection liaison officers that meet regularly across Europe to discuss good practice.
Europol has acted as a forum for promoting the more efficient and effective movement of witnesses between European states. I definitely get the sense that it's a more common practice. The procedures and practices are being developed to make that process as efficient and effective as possible.