This happens to a lot of high income people who are not especially financially inclined. They have everything on autopilot, automatic deposit, drafts and deductions, and don’t pay attention to their mail. Property tax is not recurring unless escrowed with a mortgage. So, the notices pile up on a desk or countertop until something hits the fan, certified mail, notice posted on the door by the sheriff, etc.
They are where I live.Four times a year...and due on the same four days each year.
That seems the most likely scenario to me as well.
More likely the home is owned by a corporation and they moved their office.
People with all that stuff going on have people to deal with stuff. They don’t tend to get any bills at home.