Thank you, Marcos.
I don't use a proxy server and this is a first time occurrence (with no discernible changes to either my machine or @ my ISP) . I don't have the time to learn about Wireshark right now but I do appreciate your offer towards understanding the whys of this pop-up.
For now, I accepted the unsigned update. If it becomes a continuous or continual issue, I'll have to set aside time to learn about and apply your suggestion.