cat wee

So sorry if this is a little off topic, I have 2 indoor cats who have a litter tray(obviously) the thing is, it's in the kitchen and I can randomly smell cat wee all around the house(my cats are not allowed upstairs) but it's a slight smell it every now and again but here comes the crazy part, they've not done it anywhere other than their cat tray, I know this because I have an 11 month old and constantly checking if they've done it places. When he was first born they constantly did it all over his things so I'm very very paranoid! Please help and tell me what this is about, it's getting on my nerves!