CO is sligty lighter than air. CO2 by contrast is heavier than air, so it can seep down through walls or floor. is gas capable to seep trough walls depends on whether ventillation openings are located on walls as gas can pass in ventilation system and on materials walls made from specifically quality of such materials, gas would escape easily through any cracks on the wall or floor. to be sure obtain CO meter and isolate room from external air, at least meter should be with sampling range up to 2,000ppm, if sensor reading somehow indicates that concentration raises and decreases, it's certain that gas leaking somewhere. spoiler: also if one one knows that you've died from CO in apartments odds are that upon your decomposition first responders who came to check you on neighbourhood demand due to corpse smell wil bel poisoned by gas given it doesnt leak and stays in room you died in