The pocket attaches to the inside of the garment. This allows for pockets for garments that do not come with pockets. Most importability, you can hide it inside the garment for extra security form theft.
You create a small 3in by 3in square pocket. Use a soft material like satin or silk. Sew the two squares together. Use reinforcing stitching. Add a zipper in the middle on the side of the pocket that will be the front. This could be a Velcro closure as well.
To attach to the garment, use a safety pin. Secure the pin on the inside of the hidden pocket. When the user goes to attach the pocket to any garment, they simply use the safety pin to attach to the fabric of the garment.
You can create a diverse line of hidden pockets Try different fabrics/patterns, make the pin more ornamental.
It can be marketed as extra security for small items like credit cards, ID cards, cash, jewelry when you have to take it off such as at a gym.
Good luck.