It looks like empty area as shown in the picture.
Hair Fall Out- Click to enlarge
After hair fall out in the patch there is empty space in the patch and it shows like there is a baldness on your head. People might be detect when they will see that area so its better to buy a new one or filling in it. It is created by etching on the head or using too much comb on the hair patch everyday. It happens when the knot going to be loose and hair fall starts. After severe hair fall the empty area gonna be visible.