In older versions of Blender you could mirror an object in Object mode by pressing S-X, S-Y or S-Z. This does not work anymore. In new version you press M instead. But this only works in Edit mode. In object mode M brings up the layer menu.
So how do I mirror an object in Object mode? I need this to place an empty at the mirrored position of the camera (for rendering an environment map) or if I want to reuse the same mesh in different objects of different orientation.