In all markets across Thailand and other crowded areas, you MUST be extremely diligent for pickpockets.
Pickpocket gangs both foreign and domestic, pray on tourists to steal their valuables and passports.
It is best if you never leave anything of value in your back pockets, certainly not your passport.
These gangs are very slick and unsuspecting.
Have the mindset to always know they are around.
Guard your valuables always!