References that don't show up in "Field Used By"
I've noticed recently that it's possible for a field to be referenced by certain objects in Marketo - such as Launchpoint services like LinkedIn Lead Gen, and Revenue Cycle Analytics - and yet still have the "Field Used By" list show as blank. (RCE does show a helpful error message when trying to hide/modify a field, but still isn't visible on the list.)

Are there other objects that anyone has seen reference a field without it showing in "Field Used By?"