There is a simlar post from 2009 that has been inactive for >6mths & closed but I have fresh news. It was thanks to this forum that I wasn't scammed, so I feel obliged to share my experience.
My partner & I are seasoned travellers (both business & leisure) and been to both popular and exotic places. This was my 6th trip to Bangkok in recent years and I still almost fell for it. We are also Asian, so are not as obvious as tourists in Bangkok, as Westerners are.
As my partner hadn't yet been to the touristy places in Bangkok in previous trips, on the Friday before the King's birthday we went to see the Grand Palace. On arrival at around 11am, we were walking past a quiet side gate of the palace. There were 2 middle-aged men in what appeared to be casual uniform (charcoal grey button up top and trousers, palace log sewn onto lapel) walking in past the soldier guarding the gates and they appeared to work there.
One of them, on seeing us peeking into the gate, first gestured for us to go further (to the main gate maybe?), but when we didn't move on immediately, one of them came back out and asked us where we were going, to which we replied the Palace.
He told us that the Palace was shut for a Buddhist ceremony and would only reopen at 1pm. We were unpleasantly surprised as we would have nothing to do for 2 hours. He suggested we visit the White Buddha and the Happy Buddha, and also visit the Export Centre to kill time. He made large marks on our map where these places were. When we asked what the last was, he informed us that it is only open to the public one week a year and is usually for trade, where you can get gems like earrings, rings, bracelets, etc. This day happened to be the last day, and we could find some good bargains. All this in time for us to be back by 1pm when the palace reopens.
Since none of these places are very close by he suggested we take a tuk tuk, and it would only cost THB 40 (which is ridiculously cheap even for Bangkok for the distance).
We noticed that he had a permit type of plaque pinned above his breast pocket (above the embroidered palace logo), which indicated "Tourist Police - No. 1155". We thought we bumped into a friendly and helpful tourist policeman and was quite grateful.
He started to walk away but we actually asked him to help us explain to a Tuk Tuk where we wanted to go and bargain it down to THB 40 as no doubt they would charge us more, him being a local. He gladly did so and flagged a Tuk Tuk which happened to pass by (most of them were parked on the other side of the road and few passed by earlier) and explained to the man in Thai where we wanted to go and he agreed to THB 40.
So we waved goodbye to the "Tourist Police", and he said he would "See us later when we returned, since he worked in the Palace."