Trainers,
Niantic is testing Legendary (Tier 5) Raids in the SF Bay area. At the moment, they’re using their internal “dogfood” servers (intended only for Niantic employees) and very short visibility of Tier 5 Raids to cloak their tests.
As raid test intervals are super short, it’s hard to catch them live, but two redditors have managed to do it in the past 14 days. Here’s how it looks:
Legendary Tier 5 Raid Tests
Encounter 1, reported by flipdrago, shows a number of placeholder silhouettes with 5 “boss head icons” under the raid name TEST. The heads are visible while zooming in, but also much more visible in the over-contrasted version below.

Encounter 1 with additional contrast added, Tier 5 marking visible:

Encounter 2, reported by Scooby1222, roughly 2 weeks ago:
As observed, these raids are tested at random times, only in SF Bay area. San Francisco is the location of Niantic’s US offices, so this is rather expected. We are not sure why these raids are visible for regular Trainers, but it could be a mistake (or an intentional leak) by Niantic.
This further reinforces the code we found in the 0.67.1 data mine and it’s now obvious why the Legendary Egg icon and textures were found in the 0.67.1 APK.