Exactly the same experience twice at the toll. A scam involving showing you your no plate written on their pad with explanation that it had been phoned in from a unit somewhere for some road offence.
Also "pay bt2000 or go to the station". Stood my ground and was screamed at "this Thailand, I Thai police".
Eventually got away with bt200 a a very nasty taste in my mouth.
Happened again a month later. tried the same story, but backed down when they realized my Thai missus was in the car. I guess most unsuspecting tourists would be intimated enough to pay up. Easy pickings.
I think maybe they chose me as I had Krabi plates on the vehicle .