A friend of mine had two unspayed females; the one had peed all over. When she tore up the carpet in her apartment, she could hardly believe the extent of the stuff!

So if Sterling and Lucas are otherwise healthy and neutered and all that - they could be picking up 'cues' from what is under the carpet.

It's a thought. Get a gang of friends together for pizza and carpet ripping, and get some of that industrial-strength deodorizer!

Worked wonders for my friend - and a surprise cash gift allowed her to get her cats spayed(even low-cost was expensive for her!)

Good luck!