What is this in her litterbox? 5yo female cat

Hi all, your advice appreciate.

Every few weeks i have been finding a caked mess like this in our cat's litterbox. This has happened a few times and I don't know what to make of it.

She (about 5yo, spayed, female) was treated for a urinary tract infection recently, so I was dismayed to find this mess in her letterbox today.

It doesn't smell like poo or pee or vomit. It covers a wide area, but is very shallow - it is not clumping in balls like pee.

Possibly she is vomiting water?

Any ideas what it could be and what it means for her health?

I'm posting because we \*just\* spent a fair chunk at the vet and I am reluctant to take her in again if I don't have to. I will, of course if it is the best course of action, though.