We’re seeing circles
The Bifrost is a mystical gateway that connects different realms. It always leaves a permanent mark on the ground wherever the portal opens. You could see the circular marks left on the ground in Avengers: Age of Ultron as Thor was preparing for his quest to hunt down Infinity Stones.
In Ant-Man, when the hero returns to headquarters, we can see the circular symbol that Thor left on the grass as he was returning to Asgard. The imprint can be found on the top-left corner or near the landing jet landing pad. Now that’s some crazy attention to detail!