This happened to my friend.
Whenever Safari closes and reopens she gets a small popup as follows in her iMac (OS X 10.11.6, Safari v. 11.1.2). She never clicks the button but in order to try and stop this she has cleared history, emptied caches and checked to see if it is an extension (it's not).
Is this some kind of malware? How do we get rid of it?