Today I received an SMS from ? @GMX.com
Of course I knew it was absurd, so deleted it.
At the same time 3 apps were downloading auto updates, - the only one I recall was Kii Safe, but I am not suggesting it was the cause of ......
When I went to clear the notifications and the antivirus (3 of them) message saying the downloads were clean, - a black screen came up with a large advertisement, featuring a man @ woman for a singles dating agency.
There is a menu, which allows 2 options, and that is to click for the dating service, or to skip it.
When you do the latter, the app. that was updated opens as normal.
So only when apps. are updated, or a new app. is downloaded, this advertisement appears.
It isn't an app. itself, it seems to have been bundled deliberately into an app.
I sent a message to Google security, and received a notification it had been received. As I said, the advert. only come on with app. related notifications.
So, whether that SMS installed it, or an app. update, I am unsure.
If you have an apps. with auto updates, (or manual) - you could get more than you bargained for.
I will advise when I receive a reply from Google.