I'm not completely sure about the ISY, but in most systems it's more like the latter. Trying to keep up with whether all members of this set of lights is on and at a specific level would be way too much to bother with, and scenes are just basically sending out a set of states/levels when invoked, and that's it. In some cases the device remembers the last invocation state of the scene so that you could read it, but maybe not even that.
In some systems there's no such thing as the state of a scene. There's not a list of scenes each of which can be enabled or disabled, there's just a currently active scene which can be set to one of a list of defined scenes, i.e. they are all mutually exclusive.
Generally speaking, if I was designing a system and wanted to have some hope it would still work (without massive changes) should I replace the lighting system, I wouldn't depend in sensing the state of scenes.
* But, then again, I could be completely wrong.