This was probably written by that Grisham character, right? If he wrote answers to essay exams the way he writers sentences which earn him literally millions, he'd never pass a single English composition course at any level.
Not if he followed the nonsense that was around when you were in school, eh, Merry? Luckily, he doesn't.
The real problem with this poor excuse for a sentence isn't the disagreement between the two verbs in it but, rather, the feeble attempt to describe something which the author freely admits was actually "invisible."
What disagreement, Merry?
I can't understand what you're talking about, Merry. Authors frequently describe situations for their readers that are not visible to the characters contained within.
What's his name King, not to mention pretty much every ghost/horror/murder mystery writer would be dead in the water without this ability.