I think the Non B is not primarily for work permit ... it can also be used as Business visa to conduct business in Thailand....such as import export product.. like u can use it to bring products to sell here ... So mayb Hong Kong having important business relation with Bangkok can issue Non B for the purpose of business travel
I guess it's normal for plain clothes police to suddenly appear somewhere and start asking foreigners for documents ...Saw them yesterday at Airport link.... So u are on your own.
How did such a mistake be made based on visa and ticket....So simple and and so expensive....shows u how humans can be so vulnerable to make mistakes .... I have made stupid mistakes before and tend to do sometimes .. but messing up ticket and visa dates aren't among my usual stuff to mess...and of course I am usually required for a visa for everywhere that must have trained me .... lol