The paper crane is an origami crane used as a sign of watching over someone.
Michael prominently left paper cranes in various places such as The letter including files and pictures of Fibonaci in witness protection to one of Abruzzi's henchmen, On John's table when they first met. Sara's table, Lincoln's cell and Sucre's bed.
When Michael was younger, Lincoln always left a paper crane in his room whenever he had to leave.
Michael sent Sara some coded paper cranes with coded messages to communicate.
Before Michael had a fight with World, he had a paper crane in his hand..
- Pilot (First appearance)