I'd spotted this from the road a couple of years ago, and wondered about it ever since. It certainly looked suspicious from a distance, but whenever I've passed since, a combination of rain, foot-deep mud and cows have prevented further exploration. It's not marked on any of the current (or the 1867) OS maps, and I've not seen a reference to it anywhere.
I finally got in in September - there are three stones on the corners of a rectangle. The place where the fourth should be appears to have suffered soil erosion, so the stone has maybe toppled down the hill. There are a few likely candidates in the vicinity of what is definitely a ruined four-poster.
The stones stand on a small mound on top of a big spur of land, which looks like it might have been accentuated artificially, although I think it's too big to be entirely man-made.
It reminds me of the "igloo" at Kilmorich on a larger scale, but only in shape - the majority of this one is almost certainly natural.
There are 2 larger stones, and one smaller. The 2 large ones are aligned N-S, while the smaller one is aligned with one of the larger ones E-W. The size of the rectangle is approximately 3m N-S by 1.5m E-W
Directions - Heading north on the A9 from Perth, turn off at Bankfoot on to the B867. Continue through Bankfoot, and then on through the next village of Waterloo. Approximately 1km after Waterloo, the road curves round to the right. Just as it starts to curve, you'll notice some trees on your left - the stones are in amongst the trees, just visible about 10m from the road. There's space to park off the road a little bit furtheron, just beyond the nearby Staredam stones at the loch by the Stare Dam.