I have also beginning this days receiving E-Mails with "High CPM from your device". I can fix this problem on the GMC website on my own user profile. The user profile there is a setting for set the private threshold. It was set to 30 CPM. exactly the value of the e-mail content.
Setting a private threshold on the GMC user profile sounds like a plausible solution. Aha, the threshold was suspiciously set to the exact CPM mentioned in the email! I recall once, troubleshooting a sluggish website and discovering a hidden script hogging resources; solving it felt like winning in Slope Game, a great relief. https://slopegamerun.com
I also started getting warnings on 30 cps some months ago. Did anyone find out if someone added this to our profiles, or if it is something in the environment or device which has changed causing these measurements.