The whole of Dartmoor is covered with hundreds of "letterboxes" which people hunt out and collect stamps from. Its an old custom dating back to the original one positioned at Canmore Pool, which was used to show that you have been to this extremely difficult to get to place in the middle of the moor. These days its all a bit of fun, and people put their own boxes in all sorts of places for other people to hunt down, like a sort of treasure hunt. I am sure you can easily find all sorts of details bfrom an internet search for "Dartmoor Letterbox".
However, there is a code of conduct, in that none of these boxes should be hidden or placed within the immediate vicinity of any ancient monuments, or other significant places, in order that no damage is done to them either by hiding the box there in the first place, or by people moving stones around or digging looking for them. The box you describe should certainly not be anywhere near to the centre of the Yellowmead circles. And to say it had something to do with what would appear to be a respectable organisation like Dartmoor mountain rescue team defies belief. Perhaps someone should have words with them.
Something is not right. This message is just to keep things from messing up down the road