Finders, Keepers?
Tami Robinson decided that she would single-handedly tackle the homeless problem in Florida. She located abandoned houses, filed some paperwork with the county government, and then proceeded to change the locks.
Mezuzah
Mezuzah on an Ownerless House in the Gemara
Whether someone living in a house declared hefker must affix a mezuzah, and how declaring a home ownerless was used to avoid staying without valid mezuzos.