You might not want to quibble about 9+ feet x [fence length] of land, but now that you know the fence is squatting on your
property, you risk losing ownership
of the land it encloses if you do nothing. After a certain amount
of "open and notorious" use (like, someone builds a fence on your land and uses the land behind it as theirs and you can see it), the squatter can gain ownership of the disputed land under the laws of adverse possession.