Hi Jeff,
The plan would show up in the unjoined parcels list if it contains an historic parcel that is unjoined, and the historic parcel has been filtered out from the Parcel explorer tree-view. You can turn on viewing the historic parcels by right-clicking the white-space in the parcel explorer window and checking on the option that says "View Historic Parcels" We're planning to update the software at the 10.3 release for the parcel explorer in the way that it handles historic parcel filtering, so that we can avoid this confusion in the future. Thanks for reporting the problem!
On your other question, the only valid reason that I'm aware of for the check-box to be disabled and checked on, is if the parcel is already unjoined. If you are seeing that the check-box is checked on and disabled for joined parcels, then please contact tech support, as that would be a software defect that needs to be fixed.
Thanks,
-Tim