Recap for those who don't remember or didn't read my earlier post:  I
had been having a problem with cats wetting on my living room carpet,
but I was unable to find the spot.  I can smell it when I walk in
certain areas, but couldn't see anything.

Several people recommended a black light, so I rented one.  The results
were very disappointing.  I found a few tiny spots here and there, but
nothing to amount to anything, in spite of the fact that the smell is
quite strong in one corner.  I guess the urine has soaked down and
permeated the pad.  Short of having the pad replaced, I don't know what
to do about it, other than to keep spraying with F.O.N.  If the stuff
weren't so darned expensive, I'd really drench that corner in it.

To those who recommended having a vet check the cats, I'm not ignoring
that.  I have a visitor from Australia coming in a day or two, and I'm
busily getting ready for her.  If the cats were showing any other
symptoms of a problem, I'd take the time to get them to a vet.  However,
they both act normal (for cats) and healthy, so I'm going to wait until
after her visit.

Thanks again for all the suggestions.

Joy