In July 2013 parchmarks at Stonehenge were noticed and recorded. Parchmarks are where the grass is suffering from drought stress due to a difference in the soil compared to the rest of the area. In this context sharply defined ones can be taken as an indication of an archaeologically interesting hole that has refilled with slightly looser chalk which drains better than the undisturbed.
The main observations were of four large marks which indicated where the outer stone circle could have been completed with missing stones. There were also fainter, smaller marks which seemed to indicate a ring of holes between the Z and Y holes.
The 2013 observations were the basis of a paper https://www.researchgate.net/publication/286891547_Parchmarks_at_Stonehenge_July_2013 and a commentary https://mikepitts.wordpress.com/2014/09/01/at-last-a-stonehenge-story-that-is-exactly-what-it-says/ which is a very good summary and has this plan showing the rings of marks outside the stone circle.https://www.theguardian.com/culture/2014/sep/01/stonehenge-dry-spell-grass-perfect-circle )