I read in a list of potential property title defects that there could be an "unknown easement" (#7 in the list).
How could there be an unknown easement? I thought all easements must be recorded on the deed.
Unrecorded sewer easements seem to be a major category of such problems: the easement may only be recorded on the government sewer map, and not against the property. Likewise underground gas and electric. There is also a prescriptive easement, like adverse possession, though in that case the issue would be that the property owner may not know that he was granting an easement by allowing the use of the land.