Google has been for a few months now sending text messages from Android handsets silently and without users’ explicit consent or knowledge to validate that they’re still the owners of the phone numbers they have entered into their Google account settings.
While they have explained their rationale here; it’s bizarre that that they decided to utilise users’ SMS quota without their knowledge.
These messages take the form of “(SOME_CODE) Google is verifying the phone# of this device as part of setup. Learn more: https://goo.gl/LHCS9W”.
Other forms of messages also exist.
From my observation; Google only collects these messages between midnight to 08:00am; and send those messages to phone numbers from subscribers range (e.g. in the UK, mobile subscribers ranges start with the 07 prefix).
This behaviour has also left some users confused upon the discovery of those SMS messages within their sent items, leaving them to think that their handsets have been compromised.