From time to time over the last 18 months or so, we detect a stale odor in our daughter's bedroom which is on the second floor of our house with the other bedrooms. We cannot detect where the smell is coming from or its source. We even once hired an exterminator to check our attic for a dead animal. He found nothing and said our attic was spotless. There appears to be no pattern as to when we detect the smell and the weather does not appear to play a part. We are obviously clueless and any help would be appreciated. Thanks