I have encountered this twice in BKK. Once outside central world and once outside siam paragon.
Was early so malls not opened yet, and this guy came up to tell us "it's the buddha's birthday so it's a holiday and shops open late". He then asked if we wanna go visit the biggest temple and can arrange transport for us.
The thing is that the van will bring u to buy some "diamonds" and "gems" elsewhere.
We didn't fall for it anyway but have colleagues who did. A couple was so bought by the story and even said they had been converted to become passionate about diamonds. Didn't have the heart to tell them to go verify the diamonds they bought.