Under the Assassin & Nightblade proficiencies, it shows they have access to trapfinding as a class proficiency. If neither of these classes have Find/Remove traps, why do they have this? Or was Find/Remove an omission?
It’s intended to offer an assassin some improvement to his chances of noticing a trap when he’s breaking into someone’s house to kill them.
As per the rules in Chapter 6, Traps & Trap Detection, all classes have an inherent ability to detect traps (18+). The trap-finding proficiency bonus of +2 thus increases an assassin’s chance of detecting a trap from 15% to 25%.
Note that some traps (at Judge’s discretion) might be easier to detect, e.g. a poorly concealed pit.