CS,
I have flown numerous times from UTP with Bangkok Airways and have always found them to be very professional. Please explain why you believe this is a 'private international hub' for the airline. As far as I know it only flies internally to Samui and Phuket from here.
Oh, by the way, I had a similar experience in Venice this year, I had forgotten that I'd put a bottle of water in both my and my wife's hand luggage. Hers was discovered by security, mine wasn't. They looked slightly embarassed when I handed my bottle in !!
It happens everywhere....