BEPUphysics

How can this be accomplished?

A detector volume takes a TriangleMesh in the constructor, so you can render that TriangleMesh somehow or render the model that is the source of the TriangleMesh. You may want to also set GraphicsDevice.RenderState.FillMode = FillMode.WireFrame when you draw the detector's mesh. The renderer in the demo just draws the detector's model.

What if I've created the TriangleMesh from scratch ( a set of manually created vertices and indices) and don't have a 3D model to render as a DisplayObject?

If I create a StaticTriangleGroup, I don't see a way to render that unless I also render the 3D model used to create the StaticTriangleGroup.

While the demos renderer have no built in renderer for that particular case, it is pretty similar. The TriangleMesh already has a vertex list and index list. You can convert them into a format that the graphics card likes (a VertexBuffer and IndexBuffer, for example) and just use a draw (indexed) primitives call.

Here's a little introduction to it:
http://msdn.microsoft.com/en-us/library/bb196414.aspx

There's also a lot of other resources floating around the internet, and I'm guessing you could find something in XNA creator's website samples too.