Mewtwo quite naturally moved into A-Ming's tent, casually taking off the Poké Ball belt hanging from A-Ming's waist and placing it carefully inside the backpack outside, thoughtfully zipping the bag shut.
This way, if Ah Feng wanted to sneak out in the dead of night, he would have to contend with the fragile backpack.
Barring any unexpected emergencies, Mewtwo was quite confident that big clumsy Ah Feng wouldn't be escaping; simply not wanting to ruin the backpack was a sufficiently reliable deterrent.
Chapter 173