Here is a picture of a notification Google sent to Joost de Valk from Yoast:
The notification reads as follows:
Google had detected that some pages on your site have implemented the rel-alternate-hreflang tag incorrectly. In particular, there seems to be a problem with incorrrect language and region codes, or incorrect bi-directional linking (if page A links with hreflang to page B, there must be a link back from B to A as well). Google uses the hreflang attributes to serve the correct language or regional URL to the right users in search results.
Until you correct the errors on the hreflang links, your website will not benefit from additional language or region targeting.
It was confirmed by Gary Illyes of Google that these are a brand new type of notification. Although he didn’t give a specific number of notifications that are being sent, we can probably assume it’s thousands, if not hundreds of thousands.
If you get such a notification, that means you need to resolve the issues in order to benefit from the “additional language or region targeting” offered by hreflang.