Spraying - how do I know?

We adopt our cats last May from a shelter, both have been spayed/neutered. For the past month there's been dried splatters of 'milky white' (not sure how to explain) liquid inside of Litter Robot, on one side of sofa & bookshelf (both right next to Litter Robot). The splatters don't smelled like urine. Any ideas?

Are the cats sneezing at all?

If they're spraying, it will have an odor you can't miss.

I think white-ish urine can mean a UTI.

Not that I've seen. If it isn't spraying then I wonder why he keeps going back to the same spots, even after I clean the areas w/ Nature's Miracle.

I encountered this with Oscar. I was having a snuggle with him. Then I started tapping his back, very lightly, and out shot a spray of white stuff, like you're describing. But, that was before he was neutered. If that happened now I'd be a bit concerned. Maybe take a trip to the vet and check it's not a UTI like one of the others suggests.