Welcome to the site! A couple of things...A vet visit will be the first order of business because they are usually doing these things as a way to let you know there is something wrong.
The other thing you could try because I think you indicated the one having the issue is a 17 yr old? A friend if mine had an older kitty doing the same and he change the litterbox to a long rectangular storage box that is only about 5 -6 inches high but very long in length. His kitty stopped the inappropriate urination right away and what we beleive is that the hight of the litter box or mobility issues resulted in some form of pain and the cat associates that with the litter box.
Once he replaced it everything stopped!
Tabitha April 10, 1995 - August 23, 2013
Bomber April 10, 1995 - July 12, 2010
Winston Nov 15, 1999 - September 15, 2011
Sophie Aug 30, 2011
"UNTIL ONE HAS LOVED AN ANIMAL, PART OF THEIR SOUL REMAINS UNAWAKENED"
He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion.