Aw they are so sweet.
as for your questions, I have one or 2 here that dont cover either. So i really dont think its a serious thing. Maybe just lazyness or they feel like they have to get out quickly so no one gets in there with them. I just cover it for them and hope they get the hint
as for sneezing and stuff, may have kitty colds, just keep a real close eye on them that it doesnt get worse.....

again your babies are sooooooooooooo sweet