Walked to the house with an awful smell; is it a dead animal in the walls or mildew?

I noticed a faint awful smell in part of the house yesterday. I thought maybe a mouse had died in the walls or something. It smells awful. We went out this morning and returned and the whole house smells this way. My husband says it smells like mildew. I don't know what it is or even where to begin. Nothing obvious. The AC is working fine; could there be a leak and it is mildew? The smell came on suddenly. I think its a dead rodent. Anyone have any ideas SERIOUSLY or experience with this? Thanks.