We visited in June 2016, and only followed up recently to see what the circular feature may have been - lockdown tidying up of photos!
As a result, I came across further information on Canmore, in particular a description of a 2018 study:
Geophysical Survey (16 April 2018 - 26 April 2018)
HY 37051 30722 (North Howe), HY 37274 30377 (South Howe), HY 37352 29774 (Knowe of Rowiegar), HY 37531 29661 (Knowe of Swandro), HY 37626 29296 (Westness Viking Houses), HY 4398 3224 (Rinyo) and HY 40065 27452 (Burrian)
"The RGK undertook a survey campaign on Rousay, 16 – 26 April 2018, as part of the Boyne to Brodgar Project. It aimed to provide a landscape perspective of two Neolithic core areas on the island – Westside and Rinyo. Moreover, the diachronic development in both areas will be researched, concentrating on the processes of formation and degradation of land use over time.
At Westside the survey area stretched from Westness Farm in the S to the rugged area N of North Howe, where the rocky surface and shrubbery prevented further activity (c56ha). The survey focused on the lower fields close to the coast and to the known Neolithic chambered cairns of Knowe of Swandro, Knowe of Rowiegar and Mid Howe. Some fields had to be omitted because the device could not drive across their steep slopes and rocky ground.
Several new anthropogenic anomalies and structures can be observed in the imagery. In the SE field of the surveyed area a possible new mound is most striking. A circular structure, 9.5m in diameter, of high magnetic susceptibility was evident about 250m NW of the mound.
In the N further settlement structures surrounding North Howe Broch have been revealed. Track-like linear structures follow downhill from the N towards the SW, and E of these, and NW of North Howe several possible settlement structures are evident. Three circular roundhouse structures are located 20m N of the broch, their diameter is between 8.5 and 11m. Immediately W of North Howe a concentration of stone might result from broch related debris, but could also reveal remains of further building activity in this location.
Further investigations will hopefully reveal details about the chronology and function of some of the anomaly clusters."
Something is not right. This message is just to keep things from messing up down the road