Are your smoke detectors yellow? This is not due to smoke, grease, or dirt. It’s part of the aging process when UV light causes a flame retardant in the plastic to turn the white detectors yellow. It is recommended that all detectors be replaced every 10 years. This yellow coloration is a good clue that they are around that 10 year mark.
Union Home Inspection will point out smoke detectors as old and need of replacement when necessary. This is a DIY project that only cost around $10 per detector.