First question  are there any other animals in the house  if so the cat
might be going there because it saw the other do it
Second question  how did you clean the incident before cat urine is
extremely strong in scent and is hard to remove
Here is a few links to read up on
http://www.fanciers.com/cat-faqs/general-care.shtml#litter_boxes
http://www.maxshouse.com/behavior__training_IDX.htm  this is Phil's site
great link
http://www.fanciers.com/cat-faqs/misc.shtml#removing_urine_odor

First you have to eliminate the possibility that the cat is going outside
the box because of health problems. Have the vet check the cat and explain
the behavior to him. Many times, if a health problem is found, the
inappropriate elimination stops once the health problem is addressed.

In the mean time you need to eliminate any odors odors. The cat can come
back to the spot simply because it smells that it's gone there before. NMR
gave you good links for dealing with that as well as behavior issues if it
turns out to be that.

W