Thank you all for your input.
I had a similar quandary when I was trying to convert The Sinister Secret of Saltmarsh. I wanted to have lots of illusions triggered around the house that the evil illusionist planted there to scare people away. But the kind of illusions I wanted would have been beyond the capability of that illusionist to cast. I wondered, "what happens when the characters kill the illusionist and wade through his spellbook and ask, 'How did this 3rd level illusionist create triggered illusions that lasted more than just a few minutes?'" I suppose I just like to have a self-consistent answer for those questions beyond simply "that's the way it is..."
I don't disagree that players don't need to know the answers to all the wierdness they encounter...but they will want answers to some, and it is satisfying to have a compelling explanation.
Again, thanks.
@Mortika, I like the flavor of your idea. I may use it. Thanks.