Thanks for the replies. Yes, I put the AirPods in the case when they are not being used. The problem, however, has resolved itself. Synopsis:
Switched "Play sound effects through" from "Selected Sound Output Device" to "iMac Speakers" in the Sound "Sound Effects" settings, problem went away, thought about it and figured that shouldn't have done anything, set the sound effect setting back to "Selected Sound Output Device", and the problem has stayed away.
Go figure.