Ive had cats all my life so I know what Im talking about here. Oh, and rug rats right about the kitty litter thing.
Cats wont wee in front of the door if they are stuck inside and want to get out. They will wee behind something.... like the TV (if it is a free standing one), a cabinet, between pieces of furniture, the corner of the room, behind a box. They like to feel secure when they do it.
If the cat has been weeing on the floating floor, believe me, as a non cat owner, you wouldn't have to sniff the floor to smell it!
If the cat is toilet trained, it will always use the litter tray inside (or go outside) UNLESS the litter tray is very dirty (they dont like a dirty litter tray any more than we would like a dirty toilet).
If she is otherwise a good tennant, and the only way you can tell there is a cat is by trying to smell the floor, I'd suggest that you dont have alot to worry about. Make her aware that you know, maybe get a pet bond and make sure you go over the place with a fine tooth comb when she leaves.
Rentals that allow pets are hard to get and this tennant may stay longer than others.
I have an IP rented out to a young couple (a chef and a nurse) with 2 very spoilt cats. The place is kept immaculently, they pay on time and dont whinge.
I have another that was previously rented to a woman (a nurse) with no animals, but who had 2 feral teenage kids. She didnt pay the rent for 8 weeks the first time she was breeched, 3 weeks the second time before we kicked her out, one kid was an emo and trashed his room and I had a hell of a time getting the POT smell out of the walls and carpet and she smoked continuously and I had trouble getting that smell out too.
So on paper middle aged woman with a good established job, 2 teenage kids (supposedly working) and no pets Versus 2 adults in their early 20's, just starting out in their careers with 2 cats.
Alot of land lords would dismiss the cat owners without a second thought and go for the second middle aged woman. My experience has been to give the pet owners a go IF they stack up otherwise.