Update: Yesterday I downloaded the Bitdefender Virus Scanner app and ran it to Scan Critical Locations on my iMac. Infections were found in two locations, one in Regevpop and the other in ProntoApp. The database was updated by Bitdefender and since then the dreaded pop-up/Strange message of this thread hasn't appeared. Eureka! My thanks to all contributors on this and to member Doug who recommended Bitdefender.
Roy Rainford 