Deep within the upholstery of your house are dust mites.
Your carpet, curtains, bed, and upholstered furniture's inner layers are a dust mite's paradise. Dust, or should I say your dead skin, tends to gather there. Be not ashamed; it is nearly impossible to completely eradicate dust mites from most homes. According to a study published in The Journal of Allergy and Clinical Immunology, at least one bed in about 80% of American homes had detectable levels of dust mites.
Billy has also appeared in films other than Saw. Insidious, Dead Silence, and Death Sentence all featured him as an Easter egg.