some kind of adware to buy some kind of virus protection is the likely culprit , buried itself deep in some file where it knows your scan wont find it , this in turns leads folks to buy it and it runs right to the source and turns it off .....
it is pretty safe to say this about that , most viral computer/phone issues have had the virus written , just to sell the cure
Don't know if it works on phones or not but I have had great success with Malwarebytes on my desktop. A couple times now it has found and deleted adware that other programs couldn't find.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va