We have a 19-year-old Russian Blue, who over the past few months has decided
that using the litter box is an optional activity.

Kitty was diagnosed with a benign tumor in his bladder in mid March.  The
tumor was removed and he seemed to be recovering well.  In August, he
started urinating all over the house we have lived in since April.

A full blood work up and urinalysis from the vet told us he does not have
any infection or anything, which leaves us with a behavioral cause.

We cannot trust him alone in the house as the living room carpet and various
couches are favorite targets, so over the summer we kept him in his favorite
location, the sun porch.  Now that it is colder, we have brought him inside.
My husband is in the process of building an enclosure for him, so in the
meantime, he is locked in the bathroom.

We put out a favorite blanket for him to sleep on.  He peed on it.  I tossed
it in the wash only to come back and find him sleeping in the litter box.

I looked online and couldn't find any reason why a cat would opt to sleep in
a dirty litter box.

Your thoughts?