So, my girlfriend, who is Thai, travels several times a year with her grandson (age 2 yrs.+), flying from Bangkok to visit her family upcountry. On every trip this year (3 round-trips so far), as she has done in the past, she has used her Thai passport as ID at airport security checkpoints (Don Meuang).

Today, we discovered, to our surprise (as I was preparing to make reservations for the both of us for a trip outside Thailand), that her passport expired in January, 2015. And nobody at security (or check-in) caught it.

Granted, it wouldn't be that big of a deal for her if they did catch it, as she could have used her Thai national ID card for identification to board instead. But isn't it just a little disconcerting (though probably not that surprising to many of us) how easy it is to travel with an expired passport at Thai airports?

Like you said, for inland flights in Thailand you only need an ID for identification purposes. So, an expired passport is an ID. No airport in the world checks passport validity for outward bound travelers. Usually just for ID and visas.

For domestic travel there is no need for a valid passport. So I see no connection with lax security.