Firebase crashlytics: Non-registered testers sign up

Question

I am using Firebase Crashlytics in my Android app. At this point, my app is exclusively available as an internal test version which is only visible to 3 known testers (set up within Google Play console). Everything works well and I see my crashs in the console.

However, I keep getting crash resports of totally unknown users who are not registered as testers. I can see these users in the Authentication tab in the Firebase console. Here is a snippet:

I see crash reports of totally unknown devices which definitely do not belong to any of my registered testers. They are definitely not coming from an emulator.

QUESTION: Does anyone have an idea where these users come from and how they can access an app in the internal test track?

Answer 1

OK, if anyone has the same question: I got this answer here:

Google/Android runs some automated bot tests on your app.

Would be great if that were documented somewhere...