My old cat is on his very last legs and doesn't venture out anymore, but recently, when he was just about active and he still roamed the garden, there was a time when he didn't go elsewhere beyond his own backyard and found 'sites' to make his deposits. Although he didn't use the layout ballast, he did find 'secret' areas that were near to the track etc. I had to erect thin netting to stop theses incursions.
He will be gone to the great litter tray in the sky pretty soon

but I already have the problem of other cats now claiming his territory as their own.