When Google took AMP live in their search results, they were accompanied by the AMP logo in the search results snippets. First that logo was a green color and then soon after they muted the logo to a dull gray AMP icon.
Now Google may be testing removing the AMP logo completely from the search results. Sergey Alakov spotted an example of Google showing a URL that went to an AMP page but there was no logo in the search results listings in Google.
Here is an animated GIF of the AMP logo showing in one case but gone in the other case for the same query and same URL. The last snippet, you will see the AMP logo there and then gone:
It wouldn't surprise me if Google is testing not showing the AMP logo in the search results. I mean, they use to have a mobile friendly label until they dropped that label over time.
I personally wouldn't mind if the AMP logo went away.
Forum discussion at Twitter.
Update: A Google spokesperson said "as you know we do tens of thousands of experiments in search every year but there are no such plans at this moment to change anything. The logo tells users that the page they will open will do so reliably and consistently fast."