The `shared` layout qualifier means that the layout of a particular set of variables in a block will be identical, so long as all blocks share the same definition. However, this does *not* mean that all of the members of a `shared` block are considered "active". Which means that you aren't guaranteed to be able to query the full layout for such a block from any program you compile that uses the shared block.
This is not a phantasmal issue; it is real and is really affecting code. NVIDIA's current drivers apparently will not consider shared uniforms active unless they're used. And the specification seems to allow this. See here for details: http://www.opengl.org/discussion_boards/showthread.php/181747-Nvidia-319-320-drivers-and-shared-layout-UBOs
The specification should *not* permit this.
I believe this related to something we have been recently working on internally, and will update here once that's confirmed and with the result.
The spec has been updated to consider all members of shared, std140 and std430 blocks as active, so they can be queried.