One more update:
Okay, I’ve finally identified WHICH application keeps requesting the authentication/keyring unlock. Drum roll 
…
Vivaldi.
{I stumbled upon this suspicion given that I do allow the browser to save a few innocuous logon/password combos for certain low-risk websites I’ve got bookmarked.}
And so, perusing the Vivaldi forums, I came across other users who are similarly irritated by Vivaldi’s behavior in this regard. Apparently, one can disable this, but then lose the autofill user/password feature for all websites within Vivaldi.
Although strictly speaking this isn’t necessarily a solution, at least I now know which is the offending application causing this irritating behavior.
I’ll mark this as the solution in case anyone else comes along with a similar symptom - perhaps editing the thread’s heading, too.
Thanks again to those whose assistance was offered up above. 