You should probably just check the box but if you're constrained in the # of pieces you can check, I have in fact checked a car seat with no baby before. The agent didn't blink. If you think about it, they see this all the time because often one parent is dealing with the kids while the other parents checks everybody in. I suppose she could have noticed there was no infant on the record, but it's so routine they just bagged it up and put it on the belt.