Google is testing a horizontal line about the search ads, the sponsored results, within the Google search results. We've seen Google show a blue side color on these ads, we've seen animated horizontal lines and lines separating the sitelinks in ads as well.
This newish one was first spotted by Brodie Clark on SERP Alerts and then more recently by Sachin Patel on X - here are two screenshots with different variations of this:
Forum discussion at X.
