Alright, I've really been slacking, I know!
So, to recap:
*Cats are sending a message if they fail to use their boxes
*I categorize these messages into FOUR main groupings
*Generally, the first two messages (causes) tend to be medical in nature and the latter two tend to be strictly behavioral, but there are no clear dividing lines
*What can initially be a medical cause for missing the box can soon become behavioral, through 'negative association' or by simple habit
Message 1 was about interspecific communication--cats sending a signal to other cats about their intactness/fertility or territory. If a cat is not altered at a young age, it can be difficult to rid a cat of certain behaviors, especially urine marking (or spraying). You can take care of the hormones by pulling out the testes or ovaries, but the behavior can already be ingrained. (These cats are hard to rehabilitate).
MESSAGE 2: I can't use the box.
Your cat might be telling you that it wants to use the box and would use the box, but it cannot. There could be a variety of reasons why your cat begins to eliminate inappropriately and they range from the simple-to-resolve to the more difficult:
NO ACCESS--
somewhat rare
--true lack of access (no litter pan provided, litterpan behind a closed door, etc..) Pretty simple, also not that common
--box guarded by another cat, even if subtley
--the box is a bit too far and the cat is slow/arthritic, and cannot get to where it is located
CAN'T FIND IT--
also not super common
--some old cats are semi-senile and can lose their way in a large home
--kittens may have difficulty finding the box while they are still learning
--many cats get confused if the location of their box changes
INCONTINENCE--somewhat common, usu. seen in some older cats (teens and up) or specific kinds of kitties (like manxes)
--some cats seem to simply be incontinent; examples include those with manx defect, certain old cats, cats that have had some injury (trauma to spinal cord/rear end); we currently have one cat at work that once in a while pees when mildly startled
--other cats are semi-incontinent if afflicted with a urinary tract infection or some other conditions (temporary, esp. w/ treatment)**these UTIs and 'medical' issues affecting elimination often lead to behavioral housesoiling (see Message 3)
--cats that have diarrhea due to parasites, diet, or other conditions can also become semi-incontinent
Most of the cats that are rehomed for housesoiling are not cats communicating "I cannot use the box." Rather, I tend to see mostly 'Message 3' cats and a few 'Message 4's. Those are the more interesting groups...