You mightn't see an animal, but you know it's been there because it's left its droppings, or pellets, or scats, for you to identify.
I found this on a garden chair, dried out. It was made by a possum, I think. I was hoping it was a possum and not a rat. I found a website explaining the difference between a few scats. Rats produce narrow cylindrical pellets with one or both ends pointed. I'm relieved to notice that the ends of this poo are definitely rounded, not pointed.
The poo is this picture is larger and fresher. I think this was also made by a possum, probably ringtail, taking a drink from the bird bath. It's the same shape as the one above. Ringtails eat fruit, flowers and leaves. You can see the seeds it's been eating.
Ringtail possums eat their own faecal pellets. That way they digest their food twice and make sure they get all the nutrients.