Siteimprove highlights the Google Search field to indicate it is the element it is questioning. The blinking cursor in the field does meet ADA Success Criteria.
To verify this yourself, open a new browser window and place your cursor in the URL navigation bar at the top of the screen. Tab with the keyboard until you reach the Google Search field. There you will see a blinking cursor indicating the field has focus.
Start the review of the potential issue and answer YES to "Tab to the element. Is it highlighted when reached by keyboard."
Siteimprove will indicate this is NOT a problem and move it from Potential Issues to Resolved issues.