The InstantTracker prefab from package has an InstantTrackable as its child. Is it different for you? In any case, you can remove the ImageTrackable, if that is indeed what your prefab has, and attach the InstantTrackable instead, and you will have access to the OnSceneLost event.
Title says it all. Right now I don't have a good way of determining this state.
I can't tell if the InstantTacker prefab was built wrong or not but it looks like it has the wrong kind of Trackable as its child. Comparing against the documentation it looks like it has an ImageTrackable where it should have an InstantTrackable.