do you know what kind of critter? If it's that missing hamster that you bought for the kid or something similar in size, I'd just let nature take it's course and the smell will go away in a few weeks. Other than that, I'd follow two courses of action. First, check the exterior of the house for obvious signs of intrusion, this will give you an idea of the proximity of the odorous offender. Second, think like a small animal, and inspect/follow air ducts, small cracks/holes, etc. The foul-smelling felon had to have some point of entry so finding that will help to find the curing culprit.