Child suffocation speedrun any%Rookie mistake, hiding under the covers. What you do is squeeze between the boxspring and the mattress. If you've got a decent mattress its weight should be heavy enough that it flattens out at the top and looks more or less normal, without a bulge.
Source: Have been hiding under my mattress for about 3 decades, my parents have yet to figure it out. The real trick is actually controlling your bladder so you only have to come out at night.