Do not hide the cat, whatever you do.
You will not be able to hide it. Pets leave traces - what happens if the landlord happens to drive past your house and sees the same cat in the garden repeatedly? Are you really going to hoover your house every day (or however often you'd have to do it, I'm no pet expert
) to hide the fact a cat lives there? Is your cat trained well enough? What happens when he/she invariably scratches stuff?
It will either get found or you will go insane hiding it.
Just tell your landlord and pay extra.