The smell is unlikely to be coming from the trays and surely you can narrow down a bit more where the smell is coming from? If it's coming from the downstairs loo then it will be stronger in there than in the kitchen.
If you think it's from the kitchen/utility then i think you need to go around sniffing. My first thought is that if you only have trays in the downstairs loo, what happens if the loo is in use when the cat/s want to go and so the door is shut?
They may be using something else that's on the floor in the kitchen/utility as a second place to wee. Are there cat beds there? Sniff those. Anything else on the floor?
You can also buy UV urine detectors very cheaply. You just point them around and the urine glows in the dark.