I just run W10 essentials - will look at Malware possibility
Haven't found recommended tests - just didn't know where to start looking.!
The message looks like the same as the service advising me I have Outlook Messages (lower right hand side of screen). The Outlook messages contain the words Outlook and the messages I refer to state Opera Browser. The Outlook messages are then in my Outlook mail system (having fulfilled any rules - i.e. moved,copied,deleted etc).
I have no idea where the Opera Browser messages come from and where (if) they are stored.