You may be referring to the "Obi" or sash. Obi are worn today with kimono, but the obi that goes with kosode ("small sleeves")are usually thinner than their modern counterparts. However, obi are not worn beneath kosode or kimono. Kosode are made of panels of cloth that fold over each other, making a layered undergarment.So what you might be seeing is some confusion between how the animators and or layout people treat clothing in general as the characters move.Only the model sheets might say for sure.