I've compacted the file about 3 times since noticing this error. I've learned it's best to compact frequently and I always make a backup before syncing. This error doesn't show up in FTM at all.
I've attached draft cards to someone when they are listed as the nearest relative but in this case his wife was the nearest relative.
I have just found this same problem for another person. In this case it's a death certificate for a son that I found on FamilySearch and added via FTM. It's likely that I had attached the death record to the parent at some point to use as evidence of their name but later removed it. I'm guessing the online tree has a tendency to hang onto those sources, even when they are no longer attached to any facts for that person.