Google is displaying reviews from select third party review sites in their local knowledge panels that appear in the maps results.

Conrad O’Connell was the first to spot it with reviews from Booking.com and Princeline.  But I was also able to trigger ones with Agoda as well.  Here is what it looks like:

You can slide scroll between the three different sources, although it seems go most commonly be shown with only Priceline and Booking.com.   And Booking.com always appears first.

These only appear on the local knowledge panel that displays in the map results.  They are not appearing in the organic search results, although this could change in the future.

It isn’t clear if this is a test or a rollout, but significant numbers of people are reporting seeing it.