Imagine a large round dog bed, about four feet across, stuffed with cedar shavings. The dog never actually uses it.
Now imagine you start smelling something odd near it but it's in the same general area as a litter box so you clean the litter and move on.
One day you pick up the bed to clean under it... And it's STIFF. And that letterbox smell is suddenly times a thousand.
Yep, the cats had been peeing on the dog bed, probably for MONTHS, and the cedar just kept soaking it up.
No commentary on how often we clean under dog beds.