Hi. Thanks for the reply.
I ran some tests a few months ago. The logs for those are still on my system. The dates were in April, on 22.04.2021 and 23.04.2021. Since then no new logs have been added, indicating that wherever the "staging" certs came from, it wasn't done from my location.
I can only think that the Siteground live server tested a Let's Encrpyt cert. These are automatically generated Let's Encrypt certificates for the domain, which I don't keep track of. It happens behind the scenes, but A) there should be no need for Siteground to run tests, and B) they wouldn't have been set up to send notifications to my email. Yet, somehow I am getting the messages.
This thread (link already posted above) says not to worry, nothing terrible could come of these not actually trusted certificates. Okay, I won't worry, but who could be doing this?